nir/lcssa: consider loops with no back-edge invariant

Polaris:
Totals from 6233 (4.52% of 138014) affected shaders:
SpillSGPRs: 47860 -> 48976 (+2.33%)
CodeSize: 69764704 -> 69120700 (-0.92%); split: -0.97%, +0.04%
Instrs: 13801184 -> 13594107 (-1.50%)
Cycles: 1628800928 -> 1516137888 (-6.92%)
VMEM: 910459 -> 910208 (-0.03%); split: +0.00%, -0.03%
SMEM: 436625 -> 435194 (-0.33%); split: +0.06%, -0.38%
SClause: 534750 -> 534620 (-0.02%); split: -0.03%, +0.00%
Copies: 1587121 -> 1542867 (-2.79%); split: -2.81%, +0.03%
Branches: 545016 -> 509354 (-6.54%)
PreSGPRs: 618545 -> 619354 (+0.13%); split: -0.09%, +0.22%