intel/fs: Combine constants for SEL instructions too

intel/fs: Combine constants for SEL instructions too

It is very common to have bcsel where the second and third sources are
both constants.  This results in a situation where we would want to emit
a SEL with two constant sources, but that's not allowed.

Previously, we would load both constants into registers, then let
constant propagation copy the last constant into the SEL instruction.
This results in the constant using an entire SIMD register instead of a
single channel.

Instead, copy propagate both sources, then let the combine-constants
pass do its thing.  In the worst case, this stores the constant in a
single channel of the SIMD register.  In the best case, it reuses a
value that was loaded into a register to satisfy another instruction.

shader-db results:

Tiger Lake, Ice Lake, and Skylake had similar results. (Ice Lake shown)
total instructions in shared programs: 19951549 -> 19948709 (-0.01%)
instructions in affected programs: 482795 -> 479955 (-0.59%)
helped: 1184 / HURT: 3

total cycles in shared programs: 858584724 -> 858205341 (-0.04%)
cycles in affected programs: 356168375 -> 355788992 (-0.11%)
helped: 1448 / HURT: 1195

total spills in shared programs: 6569 -> 6255 (-4.78%)
spills in affected programs: 912 -> 598 (-34.43%)
helped: 58 / HURT: 0

total fills in shared programs: 8218 -> 7813 (-4.93%)
fills in affected programs: 1570 -> 1165 (-25.80%)
helped: 58 / HURT: 0

LOST:   6
GAINED: 16

Broadwell
total instructions in shared programs: 17819660 -> 17819389 (<.01%)
instructions in affected programs: 1078129 -> 1077858 (-0.03%)
helped: 1067 / HURT: 304

total cycles in shared programs: 904722624 -> 905035016 (0.03%)
cycles in affected programs: 362583117 -> 362895509 (0.09%)
helped: 1381 / HURT: 1123

total spills in shared programs: 17884 -> 17922 (0.21%)
spills in affected programs: 5088 -> 5126 (0.75%)
helped: 55 / HURT: 152

total fills in shared programs: 25533 -> 26290 (2.96%)
fills in affected programs: 12992 -> 13749 (5.83%)
helped: 61 /HURT: 295

LOST:   7
GAINED: 24

Haswell
total instructions in shared programs: 16678080 -> 16673976 (-0.02%)
instructions in affected programs: 1162893 -> 1158789 (-0.35%)
helped: 1584 / HURT: 7

total cycles in shared programs: 880180082 -> 879932525 (-0.03%)
cycles in affected programs: 364067522 -> 363819965 (-0.07%)
helped: 1226 / HURT: 976

total spills in shared programs: 14937 -> 14428 (-3.41%)
spills in affected programs: 7866 -> 7357 (-6.47%)
helped: 351 / HURT: 5

total fills in shared programs: 17572 -> 16975 (-3.40%)
fills in affected programs: 11028 -> 10431 (-5.41%)
helped: 350 / HURT: 3

LOST:   8
GAINED: 16

Ivy Bridge
total instructions in shared programs: 15704044 -> 15703158 (<.01%)
instructions in affected programs: 304513 -> 303627 (-0.29%)
helped: 707 / HURT: 0

total cycles in shared programs: 433560149 -> 433471118 (-0.02%)
cycles in affected programs: 19299650 -> 19210619 (-0.46%)
helped: 687 / HURT: 395

LOST:   2
GAINED: 9

Sandy Bridge
total instructions in shared programs: 13913386 -> 13912884 (<.01%)
instructions in affected programs: 195687 -> 195185 (-0.26%)
helped: 455 / HURT: 0

total cycles in shared programs: 741156272 -> 741136266 (<.01%)
cycles in affected programs: 10934349 -> 10914343 (-0.18%)
helped: 578 / HURT: 289

LOST:   9
GAINED: 4

Iron Lake and GM45 had similar results. (Iron Lake shown)
total instructions in shared programs: 8364056 -> 8364042 (<.01%)
instructions in affected programs: 5178 -> 5164 (-0.27%)
helped: 10 / HURT: 0

total cycles in shared programs: 248759794 -> 248757940 (<.01%)
cycles in affected programs: 4305246 -> 4303392 (-0.04%)
helped: 183 / HURT: 24

fossil-db results:

Tiger Lake
Instructions in all programs: 156943594 -> 156802601 (-0.1%)
Instructions helped: 20595
Instructions hurt: 23248

Cycles in all programs: 7512086950 -> 7528386387 (+0.2%)
Cycles helped: 29531
Cycles hurt: 27837

Spills in all programs: 13500 -> 5643 (-58.2%)
Spills helped: 394
Spills hurt: 22

Fills in all programs: 18943 -> 6306 (-66.7%)
Fills helped: 394
Fills hurt: 11

Gained: 93
Lost: 76

Ice Lake
Instructions in all programs: 141395899 -> 141249621 (-0.1%)
Instructions helped: 30067
Instructions hurt: 3

Cycles in all programs: 9097127057 -> 9089668235 (-0.1%)
Cycles helped: 32268
Cycles hurt: 24315

Spills in all programs: 13695 -> 7564 (-44.8%)
Spills helped: 403

Fills in all programs: 18400 -> 8494 (-53.8%)
Fills helped: 403

Gained: 114
Lost: 137

Skylake
Instructions in all programs: 131948328 -> 131826063 (-0.1%)
Instructions helped: 29968
Instructions hurt: 3

Cycles in all programs: 8794778440 -> 8793934844 (-0.0%)
Cycles helped: 32705
Cycles hurt: 23575

Spills in all programs: 10526 -> 7039 (-33.1%)
Spills helped: 403

Fills in all programs: 11025 -> 7728 (-29.9%)
Fills helped: 403

Gained: 102
Lost: 250

Tested-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7698>