radv: remove exports without color attachment or writemask

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%

Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14771>