The vec2 index helps the compiler make use of SMEM's SOFFSET field when
loading descriptors.

fossil-db (GFX10.3):
Totals from 126326 (86.37% of 146267) affected shaders:
VGPRs: 4898704 -> 4899088 (+0.01%); split: -0.02%, +0.03%
SpillSGPRs: 13490 -> 14404 (+6.78%); split: -1.10%, +7.87%
CodeSize: 306442996 -> 302277700 (-1.36%); split: -1.36%, +0.01%
MaxWaves: 3277108 -> 3276624 (-0.01%); split: +0.01%, -0.02%
Instrs: 58301101 -> 57469370 (-1.43%); split: -1.43%, +0.01%
VClause: 1208270 -> 1199264 (-0.75%); split: -1.02%, +0.28%
SClause: 2517691 -> 2432744 (-3.37%); split: -3.75%, +0.38%
Copies: 3518643 -> 3161097 (-10.16%); split: -10.45%, +0.29%
Branches: 1228383 -> 1228254 (-0.01%); split: -0.12%, +0.11%
PreSGPRs: 3973880 -> 4031099 (+1.44%); split: -0.19%, +1.63%
PreVGPRs: 3831599 -> 3831707 (+0.00%)
Cycles: 1785250712 -> 1778222316 (-0.39%); split: -0.42%, +0.03%
VMEM: 52873776 -> 50663317 (-4.18%); split: +0.18%, -4.36%
SMEM: 8534270 -> 8361666 (-2.02%); split: +1.79%, -3.82%

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <mesa/mesa!9523>