nir: link time opt duplicate varyings

nir: link time opt duplicate varyings

If we are outputting the same value to more than one output
component rewrite the inputs to read from a single component.

This will allow the duplicate varying components to be optimised
away by the existing opts.

shader-db results i965 (SKL):

total instructions in shared programs: 12869230 -> 12860886 (-0.06%)
instructions in affected programs: 322601 -> 314257 (-2.59%)
helped: 3080
HURT: 8

total cycles in shared programs: 317792574 -> 317730593 (-0.02%)
cycles in affected programs: 2584925 -> 2522944 (-2.40%)
helped: 2975
HURT: 477

shader-db results radeonsi (VEGA):

SGPRS: 31576 -> 31664 (0.28 %)
VGPRS: 17484 -> 17064 (-2.40 %)
Spilled SGPRs: 184 -> 167 (-9.24 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 583340 -> 569368 (-2.40 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 6162 -> 6270 (1.75 %)
Wait states: 0 -> 0 (0.00 %)

vkpipeline-db results RADV (VEGA):

Totals from affected shaders:
SGPRS: 14880 -> 15080 (1.34 %)
VGPRS: 10872 -> 10888 (0.15 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 674016 -> 668396 (-0.83 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 2708 -> 2704 (-0.15 %)
Wait states: 0 -> 0 (0.00 %

V2: bunch of tidy ups suggested by Jason

Reviewed-by: Eric Anholt <eric@anholt.net>