aco: improve check for unreachable loop continue blocks

aco: improve check for unreachable loop continue blocks

The old code would have previously caught:
loop {
   ...
   break
}
when it was meant to just catch:
loop {
   if (...)
      break
   else
      break
}

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3658>