JIT: Clone all local addresses when importing `dup` (#72714)

Roslyn emits `dup` for the field address when compound assignment
operators are used on struct fields. We would previously spill this
address leading us to mark such structs as address exposed and disabling
promotion.

Also allow removing unnecessary casts in cases like
```
ASG
  LCL_FLD ubyte V00
  CAST int <- ubyte <- int
    ...
```

we only allowed this cast removal for LCL_VAR and IND before, which led
to unnecessary new casts in some cases with this change.

src/coreclr/jit/importer.cpp

@@ -15016,53 +15016,60 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                 tiRetVal        = se.seTypeInfo;
                 op1             = tree;
 
-                // If the expression to dup is simple, just clone it.
-                // Otherwise spill it to a temp, and reload the temp twice.
-                bool cloneExpr = false;
+                // In unoptimized code we leave the decision of
+                // cloning/creating temps up to impCloneExpr, while in
+                // optimized code we prefer temps except for some cases we know
+                // are profitable.
 
-                if (!opts.compDbgCode)
+                if (opts.OptimizationEnabled())
                 {
+                    bool clone = false;
                     // Duplicate 0 and +0.0
                     if (op1->IsIntegralConst(0) || op1->IsFloatPositiveZero())
                     {
-                        cloneExpr = true;
+                        clone = true;
                     }
                     // Duplicate locals and addresses of them
                     else if (op1->IsLocal())
                     {
-                        cloneExpr = true;
+                        clone = true;
                     }
-                    else if (op1->TypeIs(TYP_BYREF) && op1->OperIs(GT_ADDR) && op1->gtGetOp1()->IsLocal() &&
+                    else if (op1->TypeIs(TYP_BYREF, TYP_I_IMPL) && impIsAddressInLocal(op1) &&
                              (OPCODE)impGetNonPrefixOpcode(codeAddr + sz, codeEndp) != CEE_INITOBJ)
                     {
-                        cloneExpr = true;
+                        // We mark implicit byrefs with GTF_GLOB_REF (see gtNewFieldRef for why).
+                        // Avoid cloning for these.
+                        clone = (op1->gtFlags & GTF_GLOB_REF) == 0;
                     }
-                }
-                else
-                {
-                    // Always clone for debug mode
-                    cloneExpr = true;
-                }
 
-                if (!cloneExpr)
-                {
-                    const unsigned tmpNum = lvaGrabTemp(true DEBUGARG("dup spill"));
-                    impAssignTempGen(tmpNum, op1, tiRetVal.GetClassHandle(), (unsigned)CHECK_SPILL_ALL);
-                    var_types type = genActualType(lvaTable[tmpNum].TypeGet());
-                    op1            = gtNewLclvNode(tmpNum, type);
-
-                    // Propagate type info to the temp from the stack and the original tree
-                    if (type == TYP_REF)
+                    if (clone)
                     {
-                        assert(lvaTable[tmpNum].lvSingleDef == 0);
-                        lvaTable[tmpNum].lvSingleDef = 1;
-                        JITDUMP("Marked V%02u as a single def local\n", tmpNum);
-                        lvaSetClass(tmpNum, tree, tiRetVal.GetClassHandle());
+                        op2 = gtCloneExpr(op1);
                     }
-                }
+                    else
+                    {
+                        const unsigned tmpNum = lvaGrabTemp(true DEBUGARG("dup spill"));
+                        impAssignTempGen(tmpNum, op1, tiRetVal.GetClassHandle(), (unsigned)CHECK_SPILL_ALL);
+                        var_types type = genActualType(lvaTable[tmpNum].TypeGet());
 
-                op1 = impCloneExpr(op1, &op2, tiRetVal.GetClassHandle(), (unsigned)CHECK_SPILL_ALL,
-                                   nullptr DEBUGARG("DUP instruction"));
+                        // Propagate type info to the temp from the stack and the original tree
+                        if (type == TYP_REF)
+                        {
+                            assert(lvaTable[tmpNum].lvSingleDef == 0);
+                            lvaTable[tmpNum].lvSingleDef = 1;
+                            JITDUMP("Marked V%02u as a single def local\n", tmpNum);
+                            lvaSetClass(tmpNum, tree, tiRetVal.GetClassHandle());
+                        }
+
+                        op1 = gtNewLclvNode(tmpNum, type);
+                        op2 = gtNewLclvNode(tmpNum, type);
+                    }
+                }
+                else
+                {
+                    op1 = impCloneExpr(op1, &op2, tiRetVal.GetClassHandle(), (unsigned)CHECK_SPILL_ALL,
+                                       nullptr DEBUGARG("DUP instruction"));
+                }
 
                 assert(!(op1->gtFlags & GTF_GLOB_EFFECT) && !(op2->gtFlags & GTF_GLOB_EFFECT));
                 impPushOnStack(op1, tiRetVal);

src/coreclr/jit/morph.cpp

@@ -11840,7 +11840,7 @@ GenTree* Compiler::fgOptimizeCastOnAssignment(GenTreeOp* asg)
 
     GenTree* const effectiveOp1 = op1->gtEffectiveVal();
 
-    if (!effectiveOp1->OperIs(GT_IND, GT_LCL_VAR))
+    if (!effectiveOp1->OperIs(GT_IND, GT_LCL_VAR, GT_LCL_FLD))
         return asg;
 
     if (effectiveOp1->OperIs(GT_LCL_VAR) &&