aco: keep loop live-through variables spilled

aco: keep loop live-through variables spilled

fossil-db (Navi):
Totals from 3149 (2.32% of 135946) affected shaders:
VGPRs: 280928 -> 280932 (+0.00%)
SpillSGPRs: 51133 -> 30042 (-41.25%)
CodeSize: 43063076 -> 41377252 (-3.91%); split: -3.92%, +0.00%
Instrs: 8278435 -> 8037133 (-2.91%); split: -2.92%, +0.00%
Cycles: 709575456 -> 683366172 (-3.69%); split: -3.69%, +0.00%
VMEM: 542887 -> 542937 (+0.01%); split: +0.05%, -0.04%
SMEM: 210255 -> 206368 (-1.85%); split: +0.12%, -1.97%
SClause: 258847 -> 258019 (-0.32%); split: -0.52%, +0.20%
Copies: 731836 -> 684784 (-6.43%); split: -6.44%, +0.01%
Branches: 305422 -> 292844 (-4.12%); split: -4.12%, +0.00%
PreSGPRs: 333103 -> 332701 (-0.12%)
PreVGPRs: 280086 -> 280089 (+0.00%)

Helps mostly Detroit: Become Human and the single spilling Doom Eternal
shader.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Cc: 20.2 <mesa-stable>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6212>