aco/ra: don't rewrite affinities for phi operands after register assignment

aco/ra: don't rewrite affinities for phi operands after register assignment

The effect of doing so is random and not meaningful.

Totals from 52 (0.03% of 150170) affected shaders: (GFX10.3)
CodeSize: 538768 -> 538784 (+0.00%); split: -0.04%, +0.04%
Instrs: 100661 -> 100707 (+0.05%); split: -0.01%, +0.06%
Latency: 1205950 -> 1205768 (-0.02%); split: -0.07%, +0.05%
InvThroughput: 200106 -> 200040 (-0.03%); split: -0.31%, +0.28%
Copies: 5717 -> 5754 (+0.65%); split: -0.17%, +0.82%
Branches: 3153 -> 3162 (+0.29%)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12836>