nir/opt_shrink_vectors: shrink load_const properly

nir/opt_shrink_vectors: shrink load_const properly

This patch enables removal of arbitrary channels in
load_const instructions, if they are either unused or
duplicates of other channels and only used by ALU.

Totals from 692 (0.51% of 134913) affected shaders: (GFX10.3)
VGPRs: 21832 -> 21544 (-1.32%)
CodeSize: 1322016 -> 1313080 (-0.68%); split: -0.68%, +0.01%
Instrs: 243635 -> 242231 (-0.58%); split: -0.58%, +0.00%
Latency: 1856138 -> 1857237 (+0.06%); split: -0.09%, +0.15%
InvThroughput: 424298 -> 421671 (-0.62%); split: -0.62%, +0.01%
VClause: 4580 -> 4583 (+0.07%); split: -0.02%, +0.09%
SClause: 14336 -> 14354 (+0.13%); split: -0.04%, +0.17%
Copies: 8897 -> 8859 (-0.43%); split: -0.45%, +0.02%
PreSGPRs: 20439 -> 20437 (-0.01%)
PreVGPRs: 16011 -> 15907 (-0.65%); split: -0.97%, +0.32%

i915g:
total instructions in shared programs: 396471 -> 396309 (-0.04%)
instructions in affected programs: 6408 -> 6246 (-2.53%)
total const in shared programs: 56458 -> 56422 (-0.06%)
const in affected programs: 407 -> 371 (-8.85%)
LOST:   shaders/closed/steam/trine-2/fp-3.shader_test FS

r300:
total instructions in shared programs: 1164421 -> 1165059 (0.05%)
instructions in affected programs: 143981 -> 144619 (0.44%)
total temps in shared programs: 165488 -> 165497 (<.01%)
temps in affected programs: 318 -> 327 (2.83%)
total consts in shared programs: 922140 -> 921952 (-0.02%)
consts in affected programs: 12438 -> 12250 (-1.51%)

softpipe:
total instructions in shared programs: 2859978 -> 2860028 (<.01%)
instructions in affected programs: 183355 -> 183405 (0.03%)
total temps in shared programs: 517071 -> 516939 (-0.03%)
temps in affected programs: 1416 -> 1284 (-9.32%)
total imm in shared programs: 103601 -> 102767 (-0.81%)
imm in affected programs: 3928 -> 3094 (-21.23%)

Acked-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12468>