nir: fix opt_if_loop_last_continue()

nir: fix opt_if_loop_last_continue()

Rather than skipping code that looked like this:

     loop {
        ...
        if (cond) {
           do_work_1();
           continue;
        } else {
           break;
        }
        do_work_2();
     }

Previously we would turn this into:

     loop {
        ...
        if (cond) {
           do_work_1();
           continue;
        } else {
           do_work_2();
           break;
        }
     }

This was clearly wrong. This change checks for this case and makes
sure we now leave it for nir_opt_dead_cf() to clean up.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>