aco: fix WQM handling in nested loops

aco: fix WQM handling in nested loops

If on a nested loop
- the outer loop needs WQM but
- the inner loop doesn't need WQM and
- the break condition of the inner loop is computed in the outer loop
then it could happen that we transitioned to Exact before entering the inner loop
which could create an empty exec mask and lead to an infinite loop.

Fixes a GPU hang with RDR2

Cc: 20.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5518>