aco: properly recognize that s_waitcnt mitigates VMEMtoScalarWriteHazard

fossil-db (Navi):
Totals from 555 (0.41% of 135946) affected shaders:
CodeSize: 1005716 -> 1003400 (-0.23%)
Instrs: 195326 -> 194744 (-0.30%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5923>
1 file changed