ac: generate FMA for inexact instructions for radeonsi

ac: generate FMA for inexact instructions for radeonsi

NIR mostly does this already.

Totals:
SGPRS: 2588520 -> 2591784 (0.13 %)
VGPRS: 1666984 -> 1666888 (-0.01 %)
Spilled SGPRs: 4074 -> 4131 (1.40 %)
Spilled VGPRs: 38 -> 38 (0.00 %)
Private memory VGPRs: 2176 -> 2176 (0.00 %)
Scratch size: 2228 -> 2228 (0.00 %) dwords per thread
Code Size: 52726872 -> 52715468 (-0.02 %) bytes
LDS: 92 -> 92 (0.00 %) blocks
Max Waves: 479872 -> 479897 (0.01 %)
Wait states: 0 -> 0 (0.00 %)

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4696>