nir/opt_if: split ALU from Phi more aggressively

If the only user is a trivial bcsel which in a second step can be turned into a phi, this conversion is also worth it even if the previous result is not undefined or constant. Allows for some more loop unrolling or saves a few instructions.

Totals from 62 (0.04% of 139391) affected shaders (NAVI10):
SGPRs: 4976 -> 4992 (+0.32%)
VGPRs: 4408 -> 4472 (+1.45%); split: -0.45%, +1.91%
CodeSize: 453632 -> 464000 (+2.29%); split: -0.32%, +2.60%
MaxWaves: 527 -> 511 (-3.04%); split: +0.38%, -3.42%
Instrs: 84940 -> 86681 (+2.05%); split: -0.36%, +2.41%
Cycles: 11946844 -> 11783708 (-1.37%); split: -1.40%, +0.04%
VMEM: 9403 -> 10357 (+10.15%); split: +11.59%, -1.45%
SMEM: 3003 -> 3025 (+0.73%); split: +1.07%, -0.33%
VClause: 1756 -> 1997 (+13.72%); split: -0.11%, +13.84%
SClause: 2914 -> 2915 (+0.03%); split: -0.10%, +0.14%
Copies: 6426 -> 6768 (+5.32%); split: -4.14%, +9.46%
Branches: 2105 -> 2102 (-0.14%); split: -1.66%, +1.52%
PreSGPRs: 2921 -> 2909 (-0.41%); split: -0.55%, +0.14%
PreVGPRs: 4151 -> 4179 (+0.67%); split: -0.24%, +0.92%

Fixes: a No Man's Sky issue resulting in a black screen. The issue gets resolved by unrolling a loop and removing a multiplication which otherwise generates a NaN.