nir/opt_if: fix opt_if_merge when destination branch has a jump

nir/opt_if: fix opt_if_merge when destination branch has a jump

Fixes a case where opt_if_merge created code like:
if (...) {
   break;
   loop {
      ...
   }
}
which caused opt_peel_loop_initial_if to complain that the loop pre-header
wasn't a predecessor of the loop header. This patch prevents this
(invalid, I think) unreachable code from being created.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3496
Fixes: 4d3f6cb9739 ('nir: merge some basic consecutive ifs')
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6633>