radeonsi/nir: enable lowering of fpow

radeonsi/nir: enable lowering of fpow

Lowering fpow in NIR rather than LLVM can be beneficial.

Polaris results:

Totals from affected shaders:
SGPRS: 124928 -> 124896 (-0.03 %)
VGPRS: 68616 -> 68332 (-0.41 %)
Spilled SGPRs: 394 -> 413 (4.82 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 3668912 -> 3658368 (-0.29 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 18575 -> 18593 (0.10 %)
Wait states: 0 -> 0 (0.00 %)

Fixes: d6b753920677 "ac/nir: remove emission of nir_op_fpow"
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>