nir/algebraic: fold some nested bcsel

nir/algebraic: fold some nested bcsel

Totals from 14266 (10.62% of 134368) affected shaders (Polaris):
SGPRs: 761756 -> 762732 (+0.13%); split: -0.00%, +0.13%
VGPRs: 430392 -> 430924 (+0.12%); split: -0.05%, +0.17%
SpillSGPRs: 4652 -> 4628 (-0.52%); split: -0.60%, +0.09%
CodeSize: 30133000 -> 29949780 (-0.61%); split: -0.66%, +0.05%
MaxWaves: 102122 -> 102111 (-0.01%); split: +0.00%, -0.01%
Instrs: 5845085 -> 5841668 (-0.06%); split: -0.08%, +0.03%
Cycles: 69033140 -> 68889188 (-0.21%); split: -0.22%, +0.01%
VMEM: 8479021 -> 8474978 (-0.05%); split: +0.03%, -0.08%
SMEM: 831437 -> 830464 (-0.12%); split: +0.06%, -0.18%
VClause: 105411 -> 105410 (-0.00%); split: -0.01%, +0.01%
SClause: 327727 -> 327780 (+0.02%); split: -0.00%, +0.02%
Copies: 372704 -> 373306 (+0.16%); split: -0.16%, +0.32%
Branches: 112260 -> 112269 (+0.01%); split: -0.00%, +0.01%
PreSGPRs: 433308 -> 433631 (+0.07%); split: -0.01%, +0.09%
PreVGPRs: 397888 -> 397905 (+0.00%); split: -0.01%, +0.01%

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4830>