aco/ra: don't set precolor affinities for already assigned temporaries

Also don't overwrite existing precolor affinities.

Totals from 248 (0.31% of 79839) affected shaders: (Navi48)

Instrs: 154427 -> 154401 (-0.02%); split: -0.12%, +0.10%
CodeSize: 812880 -> 812568 (-0.04%); split: -0.12%, +0.08%
VGPRs: 12432 -> 12408 (-0.19%)
Latency: 851623 -> 851801 (+0.02%); split: -0.03%, +0.05%
InvThroughput: 156569 -> 156581 (+0.01%); split: -0.04%, +0.05%
VClause: 2672 -> 2681 (+0.34%); split: -0.34%, +0.67%
Copies: 12645 -> 12660 (+0.12%); split: -0.53%, +0.65%
VALU: 82894 -> 82909 (+0.02%); split: -0.08%, +0.10%
SALU: 25406 -> 25424 (+0.07%); split: -0.07%, +0.14%
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36851>

src/amd/compiler/aco_register_allocation.cpp

@@ -72,6 +72,9 @@ struct assignment {
    }
    void set_precolor_affinity(PhysReg affinity_reg)
    {
+      if (assigned || precolor_affinity)
+         return;
+
       precolor_affinity = true;
       reg = affinity_reg;
    }