radv: vectorize RT stack access

radv: vectorize RT stack access

fossil-db (gfx1100):
Totals from 10 (0.01% of 133461) affected shaders:
MaxWaves: 176 -> 174 (-1.14%)
Instrs: 39260 -> 38710 (-1.40%)
CodeSize: 202272 -> 197288 (-2.46%)
VGPRs: 888 -> 900 (+1.35%)
Latency: 82306 -> 81762 (-0.66%); split: -0.68%, +0.02%
InvThroughput: 11182 -> 11158 (-0.21%); split: -0.52%, +0.30%
VClause: 721 -> 700 (-2.91%)
SClause: 1147 -> 1148 (+0.09%); split: -0.17%, +0.26%
Copies: 3625 -> 3891 (+7.34%)
PreVGPRs: 819 -> 845 (+3.17%); split: -0.37%, +3.54%

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24350>