aco: move VMEM instructions below descriptor loads

aco: move VMEM instructions below descriptor loads

This is to prevent sequences like:
   a = descriptor_load()
   vmem(a)
   b = descriptor_load()
   vmem(b)
and instead create:
   a = descriptor_load()
   b = descriptor_load()
   vmem(a)
   vmem(b)

fossil-db (GFX10.3):
Totals from 114521 (78.30% of 146267) affected shaders:
VGPRs: 4540352 -> 4540216 (-0.00%); split: -0.03%, +0.02%
CodeSize: 289864228 -> 289114652 (-0.26%); split: -0.29%, +0.03%
MaxWaves: 2940234 -> 2940338 (+0.00%); split: +0.00%, -0.00%
Instrs: 55112418 -> 54919910 (-0.35%); split: -0.38%, +0.03%
Latency: 956528393 -> 954682011 (-0.19%); split: -0.24%, +0.05%
InvThroughput: 229280830 -> 229238107 (-0.02%); split: -0.04%, +0.02%
VClause: 1141832 -> 1139002 (-0.25%); split: -0.63%, +0.38%
SClause: 2357840 -> 2225008 (-5.63%); split: -6.01%, +0.38%
Copies: 3316040 -> 3331519 (+0.47%); split: -0.31%, +0.77%
Branches: 1187212 -> 1186919 (-0.02%); split: -0.03%, +0.01%

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/6489>