nir/lower_goto_if: Replace a tripple loop with a double loop

nir/lower_goto_if: Replace a tripple loop with a double loop

If there's some reason why this needs to be a tripple loop, I'm not
seeing it.  As far as I can tell, all the inner-most loop does is look
for the next remaining block not already in cur_level->blocks.  There's
no reason to re-walk the whole set every time just to do that.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2401>