radv: remove exports without color attachment or writemask
This lets us make use of NIR's more advanced DCE. This includes removing of CF constructs, PS inputs and VS outputs.
Totals from 1959 (1.45% of 134913) affected shaders: (GFX10.3)
VGPRs: 73464 -> 71944 (-2.07%); split: -3.79%, +1.72%
SpillSGPRs: 6 -> 0 (-inf%)
CodeSize: 4860324 -> 4675248 (-3.81%); split: -4.92%, +1.11%
LDS: 2619904 -> 2781696 (+6.18%); split: -0.37%, +6.55%
MaxWaves: 50614 -> 50852 (+0.47%); split: +1.63%, -1.16%
Instrs: 924233 -> 887836 (-3.94%); split: -5.01%, +1.07%
Latency: 5635532 -> 5418083 (-3.86%); split: -4.53%, +0.67%
InvThroughput: 1107764 -> 1077542 (-2.73%); split: -3.44%, +0.71%
VClause: 17361 -> 16163 (-6.90%); split: -8.38%, +1.47%
SClause: 31886 -> 29323 (-8.04%); split: -8.52%, +0.48%
Copies: 53529 -> 52127 (-2.62%); split: -5.30%, +2.68%
Branches: 22993 -> 22802 (-0.83%); split: -3.44%, +2.61%
PreSGPRs: 53123 -> 51395 (-3.25%); split: -3.60%, +0.35%
PreVGPRs: 59699 -> 57424 (-3.81%); split: -5.13%, +1.32%
Sometimes, the lower number of VS outputs makes RADV create culling shaders with increased code size.