message: clean up some const error reporting around promoteds
diff: @@ -134,11 +134,9 @@ pub(crate) fn codegen_constant<'tcx>(
  {
  Ok(const_val) => const_val,
  Err(_) => {
  - if promoted.is_none() {
  fx.tcx
  .sess
  .span_err(constant.span, "erroneous constant encountered");
  - }
  return crate::trap::trap_unreachable_ret_value(
  fx,
  fx.layout_of(const_.ty),
  