aco: fix kill flags on phi operands

aco: fix kill flags on phi operands

Fossil-db changes are likely due to how the CSSA pass works.
Totals from 1782 (1.31% of 136546) affected shaders (Raven):
CodeSize: 25333292 -> 25294020 (-0.16%); split: -0.16%, +0.00%
Instrs: 4916059 -> 4908218 (-0.16%); split: -0.16%, +0.00%
Latency: 282860167 -> 282707176 (-0.05%); split: -0.08%, +0.03%
InvThroughput: 136487564 -> 136394958 (-0.07%); split: -0.12%, +0.05%
VClause: 74791 -> 74795 (+0.01%)
Copies: 542115 -> 534280 (-1.45%); split: -1.48%, +0.04%
Branches: 168977 -> 168966 (-0.01%); split: -0.01%, +0.01%

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9196>