aco: allow an extra SGPR with multiple uses to be applied to VOP3

aco: allow an extra SGPR with multiple uses to be applied to VOP3

This is in a separate patch from the apply_sgprs() rewrite so that the
rewrite can be more easily tested.

pipeline-db (Navi):
Totals from affected shaders:
SGPRS: 3056 -> 3056 (0.00 %)
VGPRS: 1632 -> 1632 (0.00 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 156468 -> 156304 (-0.10 %) bytes
Max Waves: 288 -> 288 (0.00 %)
Instructions: 29510 -> 29469 (-0.14 %)

pipeline-db (Vega):
Totals from affected shaders:
SGPRS: 2984 -> 2984 (0.00 %)
VGPRS: 1616 -> 1616 (0.00 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 156132 -> 155968 (-0.11 %) bytes
Max Waves: 289 -> 289 (0.00 %)
Instructions: 29426 -> 29385 (-0.14 %)

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>