aco: allow applying two sgprs to an instruction

aco: allow applying two sgprs to an instruction

We could create VALU instructions which read two sgprs, but only if isel
created an instruction which already read one of them.

This change is in a separate patch from the apply_sgprs() rewrite so that
it can be tested if the rewrite affected anything.

pipeline-db (Navi):
Totals from affected shaders:
SGPRS: 216 -> 216 (0.00 %)
VGPRS: 64 -> 64 (0.00 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 1756 -> 1708 (-2.73 %) bytes
Max Waves: 120 -> 120 (0.00 %)
Instructions: 312 -> 300 (-3.85 %)

pipeline-db (Vega):
Totals from affected shaders:
SGPRS: 216 -> 216 (0.00 %)
VGPRS: 64 -> 64 (0.00 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 1784 -> 1736 (-2.69 %) bytes
Max Waves: 120 -> 120 (0.00 %)
Instructions: 319 -> 307 (-3.76 %)

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>