aco: combine two sgprs into a VALU if they're the same

aco: combine two sgprs into a VALU if they're the same

This was supposed to be done before but it wasn't done correctly and
everywhere.

pipeline-db (Navi):
Totals from affected shaders:
SGPRS: 784680 -> 786128 (0.18 %)
VGPRS: 574012 -> 573892 (-0.02 %)
Spilled SGPRs: 461 -> 461 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 45477088 -> 45478172 (0.00 %) bytes
Max Waves: 81294 -> 81277 (-0.02 %)
Instructions: 8657970 -> 8622483 (-0.41 %)

pipeline-db (Vega):
Totals from affected shaders:
SGPRS: 780664 -> 782072 (0.18 %)
VGPRS: 573880 -> 573760 (-0.02 %)
Spilled SGPRs: 629 -> 629 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 45445244 -> 45448340 (0.01 %) bytes
Max Waves: 81178 -> 81161 (-0.02 %)
Instructions: 8649902 -> 8614918 (-0.40 %)

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/2883>