compiler: Lower fn call arg spans down to MIR

To enable improved accuracy of diagnostics in upcoming commits.

compiler/rustc_const_eval/src/transform/validate.rs

@@ -402,7 +402,7 @@ impl<'a, 'tcx> Visitor<'tcx> for CfgChecker<'a, 'tcx> {
                     );
                 }
                 for arg in args {
-                    if let Operand::Move(place) = arg {
+                    if let Operand::Move(place) = &arg.node {
                         if is_within_packed(self.tcx, &self.body.local_decls, *place).is_some() {
                             // This is bad! The callee will expect the memory to be aligned.
                             self.fail(