nir: Silence a warning at -Og

nir: Silence a warning at -Og

This throws a curious warning:

   In file included from ../src/compiler/nir/nir.h:32,
                    from ../src/compiler/nir/nir_opt_if.c:24:
   ../src/compiler/nir/nir_opt_if.c: In function ‘opt_if_loop_last_continue’:
   ../src/compiler/glsl/list.h:415:64: warning: ‘nif’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     415 |    return !exec_list_is_empty(list) ? list->tail_sentinel.prev : NULL;
         |                                                                ^

What's going on here is not enough of the optimizer has run to be able
to prove that nif is always initialized. So just handle the "can't
happen" case as if it could.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8724>