aco: use soffset for MUBUF instructions on SI/CI

aco: use soffset for MUBUF instructions on SI/CI

pipeline-db changes for GFX7:

80310 shaders in 40472 tests
Totals:
SGPRS: 3655300 -> 3655900 (0.02 %)
VGPRS: 2677732 -> 2678324 (0.02 %)
Spilled SGPRs: 1730 -> 1730 (0.00 %)
Spilled VGPRs: 14 -> 14 (0.00 %)
Scratch size: 15540 -> 15540 (0.00 %) dwords per thread
Code Size: 136488364 -> 136106120 (-0.28 %) bytes
LDS: 1259 -> 1259 (0.00 %) blocks
Max Waves: 601039 -> 601014 (-0.00 %)

Totals from affected shaders:
SGPRS: 316312 -> 316912 (0.19 %)
VGPRS: 273844 -> 274436 (0.22 %)
Spilled SGPRs: 770 -> 770 (0.00 %)
Spilled VGPRs: 14 -> 14 (0.00 %)
Scratch size: 16 -> 16 (0.00 %) dwords per thread
Code Size: 22724904 -> 22342660 (-1.68 %) bytes
LDS: 114 -> 114 (0.00 %) blocks
Max Waves: 30861 -> 30836 (-0.08 %)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>