aco: add NUW flag

aco: add NUW flag

This (combined with a pass to actually set the corresponding NIR flags)
should help fix a lot of the regressions from the SMEM addition combining
change.

fossil-db (Navi):
Totals from 12 (0.01% of 135946) affected shaders:
CodeSize: 12376 -> 12304 (-0.58%)
Instrs: 2436 -> 2422 (-0.57%)
VMEM: 1105 -> 1096 (-0.81%)
SClause: 133 -> 130 (-2.26%)

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/2720>