Remove GTF_NO_CSE on the first FIELD_LIST arg (dotnet#57411)

The code constructing the list reuses the original
local node for the first field. Such a node is likely
to have come from an OBJ(ADDR(LCL_VAR)) tree, thus
having GTF_NO_CSE set on it, pessimizing handling of
the promoted struct field, in particular, blocking
global constant propagation.

Fix this by calling SetOperResetFlags instead of
ChangeOper. This may lose some flags, but the other
fields in the list did not have them set in any case.

src/coreclr/jit/morph.cpp

@@ -4223,7 +4223,7 @@ GenTreeCall* Compiler::fgMorphArgs(GenTreeCall* call)
                     if (fieldLclNum == varDsc->lvFieldLclStart)
                     {
                         lcl->SetLclNum(fieldLclNum);
-                        lcl->ChangeOper(GT_LCL_VAR);
+                        lcl->SetOperResetFlags(GT_LCL_VAR);
                         lcl->gtType = fieldVarDsc->TypeGet();
                         fieldLcl    = lcl;
                     }