radv,aco: don't lower some ffma instructions

radv,aco: don't lower some ffma instructions

GFX10.3 has no v_mad_f32 and we can't recombine exact ffma into a
v_fma_f32 if they're split. GFX9+ only has v_fma_f16 and no generation has
a 64-bit MAD.

fossil-db (GFX10.3):
Totals from 84040 (57.46% of 146267) affected shaders:
VGPRs: 3717256 -> 3688064 (-0.79%); split: -0.87%, +0.08%
SpillSGPRs: 10419 -> 10403 (-0.15%)
CodeSize: 263064884 -> 262442820 (-0.24%); split: -0.31%, +0.07%
MaxWaves: 2036908 -> 2038374 (+0.07%); split: +0.10%, -0.03%
Instrs: 49849448 -> 49572182 (-0.56%); split: -0.60%, +0.04%
Latency: 908130602 -> 907764246 (-0.04%); split: -0.18%, +0.14%
InvThroughput: 207051300 -> 206762704 (-0.14%); split: -0.24%, +0.10%

fossil-db (GFX10):
Totals from 2 (0.00% of 146267) affected shaders:
Latency: 8123 -> 8107 (-0.20%)

fossil-db (GFX9):
Totals from 2 (0.00% of 146401) affected shaders:
(no statistics affected)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9805>