aco: use v_fma_mix_f32 for v_fma_f32 with 2 fp16 representable, different literals

aco: use v_fma_mix_f32 for v_fma_f32 with 2 fp16 representable, different literals

We can pack two fp16 literals into one 32bit literal and use opsel to select
the correct value. Note that LLVM currently disassembles these instructions
incorrectly.

Foz-DB Navi21:
Totals from 13365 (9.91% of 134913) affected shaders:
VGPRs: 840880 -> 840016 (-0.10%); split: -0.11%, +0.01%
SpillSGPRs: 724 -> 722 (-0.28%)
CodeSize: 82439364 -> 82451336 (+0.01%); split: -0.06%, +0.08%
MaxWaves: 244858 -> 244980 (+0.05%)
Instrs: 15265976 -> 15247201 (-0.12%); split: -0.13%, +0.01%
Latency: 223316180 -> 223272495 (-0.02%); split: -0.03%, +0.02%
InvThroughput: 41981375 -> 41969917 (-0.03%); split: -0.04%, +0.01%
VClause: 266775 -> 266558 (-0.08%); split: -0.14%, +0.06%
SClause: 646602 -> 645996 (-0.09%); split: -0.16%, +0.07%
Copies: 794703 -> 776075 (-2.34%); split: -2.46%, +0.12%
Branches: 296317 -> 296316 (-0.00%)
PreSGPRs: 658796 -> 656479 (-0.35%); split: -0.35%, +0.00%
PreVGPRs: 744014 -> 743679 (-0.05%)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20587>