aco: fix fall-through test in try_remove_simple_block() with back-edges

aco: fix fall-through test in try_remove_simple_block() with back-edges

3bca0af2 enhanced empty block determination which exposed this bug and
created an infinite loop in a Guild Wars 2 shader.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Fixes: 3bca0af25dbf6d6b162463138100abb20bc1a1cc
     ('aco: ignore parallelcopies to the same register on jump threading')

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2364
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3452>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3452>