Do not discard byref zeros in native int + byref additions (#68086)

* Delete morph transform

* Reinstate assert

Author: SingleAccretion

src/coreclr/jit/gentree.h

@@ -6189,8 +6189,7 @@ struct GenTreeArrAddr : GenTreeUnOp
         , m_elemType(elemType)
         , m_firstElemOffset(firstElemOffset)
     {
-        // Temporarily disable this assert. Tracking: https://github.com/dotnet/runtime/issues/67600
-        // assert(addr->TypeIs(TYP_BYREF) || addr->IsIntegralConst(0));
+        assert(addr->TypeIs(TYP_BYREF));
         assert(((elemType == TYP_STRUCT) && (elemClassHandle != NO_CLASS_HANDLE)) ||
                (elemClassHandle == NO_CLASS_HANDLE));
 

src/coreclr/jit/morph.cpp

@@ -13074,9 +13074,8 @@ GenTree* Compiler::fgOptimizeAddition(GenTreeOp* add)
         DEBUG_DESTROY_NODE(constTwo);
     }
 
-    // Fold (x + 0) - given it won't change the tree type to TYP_REF.
-    // TODO-Bug: this code will lose the GC-ness of a tree like "native int + byref(0)".
-    if (op2->IsIntegralConst(0) && ((add->TypeGet() == op1->TypeGet()) || !op1->TypeIs(TYP_REF)))
+    // Fold (x + 0) - given it won't change the tree type.
+    if (op2->IsIntegralConst(0) && (genActualType(add) == genActualType(op1)))
     {
         if (op2->IsCnsIntOrI() && varTypeIsI(op1))
         {