JIT: use explicit cast away GC when passing byref to unmanaged callees (

#107811)

Revise the fix done in #39105 to make the cast from byref to native int via
an explicit cast, rather than retyping the node.

Fixes #107045.

src/coreclr/jit/importercalls.cpp

@@ -2046,9 +2046,7 @@ void Compiler::impPopArgsForUnmanagedCall(GenTreeCall* call, CORINFO_SIG_INFO* s
 //    call - The call
 //
 // Remarks:
-//   This makes use of the fact that TYP_I_IMPL <-> TYP_BYREF casts are
-//   implicit in JIT IR, allowing us to change the types directly without
-//   inserting a cast node.
+//   Make the "casting away" of GC explicit here instead of retyping.
 //
 void Compiler::impRetypeUnmanagedCallArgs(GenTreeCall* call)
 {
@@ -2059,15 +2057,16 @@ void Compiler::impRetypeUnmanagedCallArgs(GenTreeCall* call)
         // We should not be passing gc typed args to an unmanaged call.
         if (varTypeIsGC(argNode->TypeGet()))
         {
-            // Tolerate byrefs by retyping to native int.
+            // Tolerate byrefs by casting to native int.
             //
             // This is needed or we'll generate inconsistent GC info
             // for this arg at the call site (gc info says byref,
             // pinvoke sig says native int).
             //
             if (argNode->TypeGet() == TYP_BYREF)
             {
-                argNode->ChangeType(TYP_I_IMPL);
+                GenTree* cast = gtNewCastNode(TYP_I_IMPL, argNode, false, TYP_I_IMPL);
+                arg.SetEarlyNode(cast);
             }
             else
             {