aco/optimizer: copy propagate to output modifier instructions

aco/optimizer: copy propagate to output modifier instructions

Foz-DB Navi21:
Totals from 847 (1.11% of 76572) affected shaders:
Instrs: 2331245 -> 2330335 (-0.04%); split: -0.04%, +0.00%
CodeSize: 12451040 -> 12451736 (+0.01%); split: -0.00%, +0.01%
Latency: 26230953 -> 26229153 (-0.01%); split: -0.01%, +0.00%
InvThroughput: 6297802 -> 6296788 (-0.02%); split: -0.02%, +0.00%
VClause: 64527 -> 64528 (+0.00%); split: -0.00%, +0.01%
SClause: 73150 -> 73121 (-0.04%); split: -0.06%, +0.02%
Copies: 180083 -> 179172 (-0.51%); split: -0.53%, +0.02%
PreSGPRs: 62311 -> 62316 (+0.01%)
PreVGPRs: 51720 -> 51710 (-0.02%)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25347>