nir/opt_algebraic: Optimize d3d9 pow with fmulz.

Foz-DB Navi21:
Totals from 69 (0.05% of 134913) affected shaders:
CodeSize: 255684 -> 253788 (-0.74%); split: -0.74%, +0.00%
Instrs: 46307 -> 46052 (-0.55%); split: -0.55%, +0.00%
Latency: 533255 -> 530742 (-0.47%); split: -0.48%, +0.01%
InvThroughput: 110001 -> 109156 (-0.77%)
VClause: 839 -> 844 (+0.60%); split: -1.19%, +1.79%
SClause: 1411 -> 1395 (-1.13%)
Copies: 1828 -> 1816 (-0.66%); split: -1.09%, +0.44%
PreSGPRs: 2243 -> 2232 (-0.49%)
PreVGPRs: 2213 -> 2192 (-0.95%)