aco: Don't verify branch exec read when eliminating exec writes.

aco: Don't verify branch exec read when eliminating exec writes.

Verifying that the branch instruction reads exec is not actually
necessary because the pattern that we look for already implies that.

This prepares for the next commit which will remove the exec operand
from branches that have the same target. These branches will no
longer read exec, but they should still get the same optimization.

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21493>