Fix recent IndexOf regressions (#80779)

* Improve IndexOf codegen for non-char types

* Call directly into NonPackedIndexOf where it makes sense

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.Char.cs

@@ -40,7 +40,8 @@ public static int IndexOf(ref char searchSpace, int searchSpaceLength, ref char
             while (remainingSearchSpaceLength > 0)
             {
                 // Do a quick search for the first element of "value".
-                int relativeIndex = IndexOfChar(ref Unsafe.Add(ref searchSpace, offset), valueHead, remainingSearchSpaceLength);
+                // Using the non-packed variant as the input is short and would not benefit from the packed implementation.
+                int relativeIndex = NonPackedIndexOfChar(ref Unsafe.Add(ref searchSpace, offset), valueHead, remainingSearchSpaceLength);
                 if (relativeIndex < 0)
                     break;
 

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.Packed.cs

@@ -24,11 +24,14 @@ internal static partial class PackedSpanHelpers
 
         // Not all values can benefit from packing the searchSpace. See comments in PackSources below.
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static unsafe bool CanUsePackedIndexOf<T>(T value) =>
-            PackedIndexOfIsSupported &&
-            RuntimeHelpers.IsBitwiseEquatable<T>() &&
-            sizeof(T) == sizeof(ushort) &&
-            *(ushort*)&value - 1u < 254u;
+        public static unsafe bool CanUsePackedIndexOf<T>(T value)
+        {
+            Debug.Assert(PackedIndexOfIsSupported);
+            Debug.Assert(RuntimeHelpers.IsBitwiseEquatable<T>());
+            Debug.Assert(sizeof(T) == sizeof(ushort));
+
+            return *(ushort*)&value - 1u < 254u;
+        }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static int IndexOf(ref char searchSpace, char value, int length) =>

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs

@@ -1306,7 +1306,7 @@ public static int SequenceCompareTo<T>(ref T first, int firstLength, ref T secon
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static unsafe bool ContainsValueType<T>(ref T searchSpace, T value, int length) where T : struct, INumber<T>
         {
-            if (PackedSpanHelpers.PackedIndexOfIsSupported && PackedSpanHelpers.CanUsePackedIndexOf(value))
+            if (PackedSpanHelpers.PackedIndexOfIsSupported && typeof(T) == typeof(short) && PackedSpanHelpers.CanUsePackedIndexOf(value))
             {
                 return PackedSpanHelpers.Contains(ref Unsafe.As<T, short>(ref searchSpace), *(short*)&value, length);
             }
@@ -1435,6 +1435,10 @@ public static int SequenceCompareTo<T>(ref T first, int firstLength, ref T secon
         internal static int IndexOfChar(ref char searchSpace, char value, int length)
             => IndexOfValueType(ref Unsafe.As<char, short>(ref searchSpace), (short)value, length);
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int NonPackedIndexOfChar(ref char searchSpace, char value, int length) =>
+            NonPackedIndexOfValueType<short, DontNegate<short>>(ref Unsafe.As<char, short>(ref searchSpace), (short)value, length);
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static int IndexOfValueType<T>(ref T searchSpace, T value, int length) where T : struct, INumber<T>
             => IndexOfValueType<T, DontNegate<T>>(ref searchSpace, value, length);
@@ -1448,7 +1452,7 @@ internal static int IndexOfChar(ref char searchSpace, char value, int length)
             where TValue : struct, INumber<TValue>
             where TNegator : struct, INegator<TValue>
         {
-            if (PackedSpanHelpers.PackedIndexOfIsSupported && PackedSpanHelpers.CanUsePackedIndexOf(value))
+            if (PackedSpanHelpers.PackedIndexOfIsSupported && typeof(TValue) == typeof(short) && PackedSpanHelpers.CanUsePackedIndexOf(value))
             {
                 return typeof(TNegator) == typeof(DontNegate<short>)
                     ? PackedSpanHelpers.IndexOf(ref Unsafe.As<TValue, char>(ref searchSpace), *(char*)&value, length)
@@ -1605,7 +1609,7 @@ internal static int IndexOfAnyChar(ref char searchSpace, char value0, char value
             where TValue : struct, INumber<TValue>
             where TNegator : struct, INegator<TValue>
         {
-            if (PackedSpanHelpers.PackedIndexOfIsSupported && PackedSpanHelpers.CanUsePackedIndexOf(value0) && PackedSpanHelpers.CanUsePackedIndexOf(value1))
+            if (PackedSpanHelpers.PackedIndexOfIsSupported && typeof(TValue) == typeof(short) && PackedSpanHelpers.CanUsePackedIndexOf(value0) && PackedSpanHelpers.CanUsePackedIndexOf(value1))
             {
                 return typeof(TNegator) == typeof(DontNegate<short>)
                     ? PackedSpanHelpers.IndexOfAny(ref Unsafe.As<TValue, char>(ref searchSpace), *(char*)&value0, *(char*)&value1, length)
@@ -1782,7 +1786,7 @@ internal static int IndexOfAnyChar(ref char searchSpace, char value0, char value
             where TValue : struct, INumber<TValue>
             where TNegator : struct, INegator<TValue>
         {
-            if (PackedSpanHelpers.PackedIndexOfIsSupported && PackedSpanHelpers.CanUsePackedIndexOf(value0) && PackedSpanHelpers.CanUsePackedIndexOf(value1) && PackedSpanHelpers.CanUsePackedIndexOf(value2))
+            if (PackedSpanHelpers.PackedIndexOfIsSupported && typeof(TValue) == typeof(short) && PackedSpanHelpers.CanUsePackedIndexOf(value0) && PackedSpanHelpers.CanUsePackedIndexOf(value1) && PackedSpanHelpers.CanUsePackedIndexOf(value2))
             {
                 return typeof(TNegator) == typeof(DontNegate<short>)
                     ? PackedSpanHelpers.IndexOfAny(ref Unsafe.As<TValue, char>(ref searchSpace), *(char*)&value0, *(char*)&value1, *(char*)&value2, length)
@@ -3085,7 +3089,7 @@ internal static int IndexOfAnyExceptInRangeUnsignedNumber<T>(ref T searchSpace,
             where T : struct, IUnsignedNumber<T>, IComparisonOperators<T, T, bool>
             where TNegator : struct, INegator<T>
         {
-            if (PackedSpanHelpers.PackedIndexOfIsSupported && PackedSpanHelpers.CanUsePackedIndexOf(lowInclusive) && PackedSpanHelpers.CanUsePackedIndexOf(highInclusive) && highInclusive >= lowInclusive)
+            if (PackedSpanHelpers.PackedIndexOfIsSupported && typeof(T) == typeof(ushort) && PackedSpanHelpers.CanUsePackedIndexOf(lowInclusive) && PackedSpanHelpers.CanUsePackedIndexOf(highInclusive) && highInclusive >= lowInclusive)
             {
                 ref char charSearchSpace = ref Unsafe.As<T, char>(ref searchSpace);
                 char charLowInclusive = *(char*)&lowInclusive;

src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs

@@ -1074,15 +1074,32 @@ public string Replace(string oldValue, string? newValue)
                 // Find all occurrences of the oldValue character.
                 char c = oldValue[0];
                 int i = 0;
-                while (true)
+
+                if (PackedSpanHelpers.PackedIndexOfIsSupported && PackedSpanHelpers.CanUsePackedIndexOf(c))
                 {
-                    int pos = SpanHelpers.IndexOfChar(ref Unsafe.Add(ref _firstChar, i), c, Length - i);
-                    if (pos < 0)
+                    while (true)
                     {
-                        break;
+                        int pos = PackedSpanHelpers.IndexOf(ref Unsafe.Add(ref _firstChar, i), c, Length - i);
+                        if (pos < 0)
+                        {
+                            break;
+                        }
+                        replacementIndices.Append(i + pos);
+                        i += pos + 1;
+                    }
+                }
+                else
+                {
+                    while (true)
+                    {
+                        int pos = SpanHelpers.NonPackedIndexOfChar(ref Unsafe.Add(ref _firstChar, i), c, Length - i);
+                        if (pos < 0)
+                        {
+                            break;
+                        }
+                        replacementIndices.Append(i + pos);
+                        i += pos + 1;
                     }
-                    replacementIndices.Append(i + pos);
-                    i += pos + 1;
                 }
             }
             else