Clarify a comment for DecisionDagBuilder.TranslatePattern.

Improve handling of array builder in in LocalRewriter.LowerIsPattern

src/Compilers/CSharp/Portable/Binder/DecisionDagBuilder.cs

@@ -26,7 +26,7 @@ internal DecisionDagBuilder(CSharpCompilation compilation)
         }
 
         /// <summary>
-        /// Used to translate the pattern of an is-pattern expression.
+        /// Used to translate the pattern of an is-pattern expression. Returns the BoundDagTemp used to represent the root (input).
         /// </summary>
         public BoundDagTemp TranslatePattern(
             BoundExpression input,

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_Patterns.cs

@@ -289,15 +289,16 @@ public BoundExpression LowerIsPattern(BoundPattern pattern, CSharpCompilation co
                     result = (result == null) ? conjunct : _factory.LogicalAnd(result, conjunct);
                 }
 
-                var bindingsBuilder = ArrayBuilder<BoundExpression>.GetInstance();
+                var bindingsBuilder = ArrayBuilder<BoundExpression>.GetInstance(bindings.Length);
                 foreach ((BoundExpression left, BoundDagTemp right) in bindings)
                 {
                     bindingsBuilder.Add(_factory.AssignmentExpression(left, _tempAllocator.GetTemp(right)));
                 }
 
-                if (bindingsBuilder.Count > 0)
+                var bindingAssignments = bindingsBuilder.ToImmutableAndFree();
+                if (bindingAssignments.Length > 0)
                 {
-                    BoundSequence c = _factory.Sequence(ImmutableArray<LocalSymbol>.Empty, bindingsBuilder.ToImmutableAndFree(), _factory.Literal(true));
+                    BoundSequence c = _factory.Sequence(ImmutableArray<LocalSymbol>.Empty, bindingAssignments, _factory.Literal(true));
                     result = (result == null) ? c : (BoundExpression)_factory.LogicalAnd(result, c);
                 }
                 else if (result == null)