nir/opt_if: don't mark progress if nothing changes

if we have something like this:

loop {
   ...
   if x {
      break;
   } else {
      continue;
   }
}

opt_if_loop_last_continue returns true marking progress allthough nothing
changes.

Fixes: 5921a19d "nir: add if opt opt_if_loop_last_continue()"
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 7e08f22a)