aco: use nir_opt_uniform_atomics

aco: use nir_opt_uniform_atomics

Significantly improves performance of a Control compute shader. Also seems
to increase FPS at the very start of the game by ~9% (RX 580, 1080p,
medium settings, no MSAA).

fossil-db (Navi):
Totals from 315 (0.23% of 135946) affected shaders:
SGPRs: 18296 -> 18336 (+0.22%); split: -0.26%, +0.48%
VGPRs: 11856 -> 11844 (-0.10%); split: -0.81%, +0.71%
CodeSize: 2233800 -> 2457508 (+10.01%)
MaxWaves: 4506 -> 4497 (-0.20%); split: +0.04%, -0.24%
Instrs: 438766 -> 486215 (+10.81%); split: -0.00%, +10.81%
Cycles: 7880180 -> 8963340 (+13.75%); split: -0.00%, +13.75%

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/6558>