ac/nir: Lower large indirect variables to scratch

ac/nir: Lower large indirect variables to scratch

results from radeonsi NIR:

Totals from affected shaders:
SGPRS: 704 -> 464 (-34.09 %)
VGPRS: 2056 -> 672 (-67.32 %)
Spilled SGPRs: 24 -> 0 (-100.00 %)
Spilled VGPRs: 28406 -> 0 (-100.00 %)
Private memory VGPRs: 0 -> 3182 (0.00 %)
Scratch size: 1064 -> 3228 (203.38 %) dwords per thread
Code Size: 935260 -> 40180 (-95.70 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 28 -> 70 (150.00 %)
Wait states: 0 -> 0 (0.00 %)

results from radv:

Totals from affected shaders:
SGPRS: 80 -> 48 (-40.00 %)
VGPRS: 204 -> 108 (-47.06 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 256 (0.00 %) dwords per thread
Code Size: 15792 -> 9504 (-39.82 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 1 -> 2 (100.00 %)
Wait states: 0 -> 0 (0.00 %)

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>