radv,aco: lower image descriptor loads in NIR

radv,aco: lower image descriptor loads in NIR

fossil-db (Sienna Cichlid):
Totals from 2926 (1.80% of 162293) affected shaders:
Instrs: 2315110 -> 2306644 (-0.37%); split: -0.37%, +0.00%
CodeSize: 12581592 -> 12546588 (-0.28%); split: -0.28%, +0.00%
VGPRs: 130216 -> 130208 (-0.01%)
SpillSGPRs: 477 -> 474 (-0.63%); split: -5.03%, +4.40%
Latency: 29686188 -> 29678804 (-0.02%); split: -0.05%, +0.02%
InvThroughput: 6926545 -> 6926286 (-0.00%); split: -0.02%, +0.02%
SClause: 73761 -> 72996 (-1.04%); split: -1.16%, +0.12%
Copies: 144068 -> 137279 (-4.71%); split: -4.78%, +0.07%
Branches: 47466 -> 47483 (+0.04%); split: -0.01%, +0.04%
PreSGPRs: 118042 -> 117377 (-0.56%); split: -1.34%, +0.77%

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12773>