radv: always consider some outputs as invariant

Now that we try to create ffma from the beginning and are less reliant on
inexact optimizations, we can enable this with much less regressions on
GFX10.3.

fossil-db (GFX10.3):
Totals from 53274 (36.42% of 146267) affected shaders:
VGPRs: 1979216 -> 1975944 (-0.17%); split: -0.67%, +0.50%
SpillSGPRs: 1462 -> 1446 (-1.09%)
CodeSize: 77389256 -> 79231200 (+2.38%); split: -0.06%, +2.44%
MaxWaves: 1418040 -> 1419400 (+0.10%); split: +0.34%, -0.24%
Instrs: 15015014 -> 15453979 (+2.92%); split: -0.14%, +3.06%
Latency: 89229974 -> 89562172 (+0.37%); split: -0.76%, +1.13%
InvThroughput: 15488878 -> 15990928 (+3.24%); split: -0.25%, +3.50%

fossil-db (GFX10):
Totals from 12563 (8.59% of 146267) affected shaders:
VGPRs: 532880 -> 532200 (-0.13%); split: -0.42%, +0.29%
SpillSGPRs: 1462 -> 1446 (-1.09%)
CodeSize: 30125368 -> 30275508 (+0.50%); split: -0.06%, +0.56%
MaxWaves: 327120 -> 327484 (+0.11%); split: +0.31%, -0.20%
Instrs: 5783753 -> 5834581 (+0.88%); split: -0.02%, +0.89%
Latency: 29271625 -> 29349407 (+0.27%); split: -0.14%, +0.41%
InvThroughput: 6801730 -> 6855695 (+0.79%); split: -0.04%, +0.83%

fossil-db (GFX9):
Totals from 12563 (8.58% of 146401) affected shaders:
SGPRs: 717840 -> 717888 (+0.01%); split: -0.16%, +0.16%
VGPRs: 531464 -> 531892 (+0.08%); split: -0.23%, +0.31%
SpillSGPRs: 1770 -> 1750 (-1.13%)
CodeSize: 27658752 -> 27814028 (+0.56%); split: -0.05%, +0.61%
MaxWaves: 81149 -> 81224 (+0.09%); split: +0.24%, -0.14%
Instrs: 5489024 -> 5537939 (+0.89%); split: -0.02%, +0.91%
Latency: 43480815 -> 43657720 (+0.41%); split: -0.08%, +0.49%
InvThroughput: 19853380 -> 20017416 (+0.83%); split: -0.04%, +0.87%

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>