radv: enable lowering of fpow to fexp2 and flog2

radv: enable lowering of fpow to fexp2 and flog2

There is no fpow in hardware, so it's always lowered somewhere,
but it appears that lowering at NIR level is better. Figured while
comparing compute shaders between RadeonSI and RADV.

Polaris10:
Totals from affected shaders:
SGPRS: 18936 -> 18904 (-0.17 %)
VGPRS: 12240 -> 12220 (-0.16 %)
Spilled SGPRs: 2809 -> 2809 (0.00 %)
Code Size: 718116 -> 719848 (0.24 %) bytes
Max Waves: 1409 -> 1410 (0.07 %)

Vega10:
Totals from affected shaders:
SGPRS: 18392 -> 18392 (0.00 %)
VGPRS: 12008 -> 11920 (-0.73 %)
Spilled SGPRs: 3001 -> 2981 (-0.67 %)
Code Size: 777444 -> 778788 (0.17 %) bytes
Max Waves: 1503 -> 1504 (0.07 %)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>