aco: mark phi definitions as last-seen phi operands

aco: mark phi definitions as last-seen phi operands

Totals from 14340 (11.23% of 127638) affected shaders:
SGPRs: 1251648 -> 1251512 (-0.01%)
VGPRs: 994556 -> 994104 (-0.05%); split: -0.06%, +0.01%
CodeSize: 122894528 -> 121099604 (-1.46%); split: -1.49%, +0.03%
MaxWaves: 106039 -> 106103 (+0.06%); split: +0.06%, -0.00%
Instrs: 23860066 -> 23414317 (-1.87%); split: -1.90%, +0.03%
Copies: 2448228 -> 2049305 (-16.29%); split: -16.37%, +0.07%
Branches: 789381 -> 757921 (-3.99%); split: -4.62%, +0.64%

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/4990>