aco/ra: try more aggressive to assign phi defs the same register

aco/ra: try more aggressive to assign phi defs the same register

Totals from 4158 (2.77% of 150170) affected shaders: (GFX10.3)
VGPRs: 312008 -> 312000 (-0.00%)
CodeSize: 42902064 -> 42892200 (-0.02%); split: -0.06%, +0.04%
Instrs: 8086443 -> 8084532 (-0.02%); split: -0.07%, +0.05%
Latency: 138551153 -> 138215222 (-0.24%); split: -0.28%, +0.03%
InvThroughput: 39676773 -> 39570850 (-0.27%); split: -0.29%, +0.02%
SClause: 306299 -> 306284 (-0.00%); split: -0.01%, +0.00%
Copies: 552481 -> 553353 (+0.16%); split: -0.75%, +0.91%
Branches: 284381 -> 282409 (-0.69%); split: -0.74%, +0.04%

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