intel/vec4: Emit constants for some ALU sources as immediate values

intel/vec4: Emit constants for some ALU sources as immediate values

In some cases of flow control, the constant propagation is not able to
determine that the source of an instruction must be a constant value.
When we still have NIR SSA values, we can easily determine this.  Emit
the immediate value during code generation to possible avoid spurious
loads of constants into registers.

I wrote this patch to prevent a couple trivial regressions in vec4
shaders caused by "nir/algebraic: Replace i2b used by bcsel or
if-statement with comparison".  The final result was quite a bit better
than that...

No shader-db changes on any Gen8+ platform.

v2: Assert that we never get a negation source modifier on Gen8+.
Suggested by Ken.  This should never happen because we don't normally
use vec4 for Gen8+ (requires and environment variable to force it), and
there's no code to generate these negations.  Still, erring on the side
of caution is better.

Haswell
total instructions in shared programs: 13776218 -> 13764783 (-0.08%)
instructions in affected programs: 663931 -> 652496 (-1.72%)
helped: 3495
HURT: 1
helped stats (abs) min: 1 max: 30 x̄: 3.28 x̃: 2
helped stats (rel) min: 0.21% max: 10.00% x̄: 1.79% x̃: 1.49%
HURT stats (abs)   min: 24 max: 24 x̄: 24.00 x̃: 24
HURT stats (rel)   min: 12.24% max: 12.24% x̄: 12.24% x̃: 12.24%
95% mean confidence interval for instructions value: -3.39 -3.15
95% mean confidence interval for instructions %-change: -1.84% -1.75%
Instructions are helped.

total cycles in shared programs: 386818984 -> 386511910 (-0.08%)
cycles in affected programs: 20379636 -> 20072562 (-1.51%)
helped: 3052
HURT: 476
helped stats (abs) min: 2 max: 12516 x̄: 110.40 x̃: 6
helped stats (rel) min: 0.05% max: 24.68% x̄: 1.58% x̃: 0.69%
HURT stats (abs)   min: 2 max: 416 x̄: 62.76 x̃: 24
HURT stats (rel)   min: 0.10% max: 10.75% x̄: 4.03% x̃: 2.18%
95% mean confidence interval for cycles value: -115.57 -58.51
95% mean confidence interval for cycles %-change: -0.93% -0.73%
Cycles are helped.

total spills in shared programs: 100482 -> 100480 (<.01%)
spills in affected programs: 79 -> 77 (-2.53%)
helped: 3
HURT: 1

total fills in shared programs: 96883 -> 96877 (<.01%)
fills in affected programs: 85 -> 79 (-7.06%)
helped: 4
HURT: 0

Ivy Bridge
total instructions in shared programs: 12000562 -> 11990113 (-0.09%)
instructions in affected programs: 572581 -> 562132 (-1.82%)
helped: 3106
HURT: 0
helped stats (abs) min: 1 max: 30 x̄: 3.36 x̃: 2
helped stats (rel) min: 0.21% max: 10.00% x̄: 1.86% x̃: 1.49%
95% mean confidence interval for instructions value: -3.49 -3.23
95% mean confidence interval for instructions %-change: -1.91% -1.81%
Instructions are helped.

total cycles in shared programs: 180958504 -> 180664500 (-0.16%)
cycles in affected programs: 19991810 -> 19697806 (-1.47%)
helped: 2654
HURT: 486
helped stats (abs) min: 2 max: 12516 x̄: 121.61 x̃: 6
helped stats (rel) min: 0.05% max: 20.66% x̄: 1.48% x̃: 0.68%
HURT stats (abs)   min: 2 max: 396 x̄: 59.18 x̃: 24
HURT stats (rel)   min: 0.05% max: 9.62% x̄: 3.82% x̃: 2.16%
95% mean confidence interval for cycles value: -125.62 -61.64
95% mean confidence interval for cycles %-change: -0.76% -0.56%
Cycles are helped.

Sandy Bridge
total instructions in shared programs: 10842336 -> 10835438 (-0.06%)
instructions in affected programs: 395340 -> 388442 (-1.74%)
helped: 1926
HURT: 0
helped stats (abs) min: 1 max: 22 x̄: 3.58 x̃: 2
helped stats (rel) min: 0.10% max: 9.68% x̄: 1.78% x̃: 1.42%
95% mean confidence interval for instructions value: -3.73 -3.43
95% mean confidence interval for instructions %-change: -1.84% -1.72%
Instructions are helped.

total cycles in shared programs: 154590074 -> 154569050 (-0.01%)
cycles in affected programs: 8159932 -> 8138908 (-0.26%)
helped: 1670
HURT: 228
helped stats (abs) min: 2 max: 260 x̄: 18.13 x̃: 6
helped stats (rel) min: 0.02% max: 8.70% x̄: 0.74% x̃: 0.28%
HURT stats (abs)   min: 2 max: 1798 x̄: 40.58 x̃: 14
HURT stats (rel)   min: 0.03% max: 12.97% x̄: 1.04% x̃: 0.31%
95% mean confidence interval for cycles value: -13.51 -8.64
95% mean confidence interval for cycles %-change: -0.60% -0.46%
Cycles are helped.

Iron Lake and GM45 had similar results. (Iron Lake shown)
total instructions in shared programs: 8212357 -> 8206587 (-0.07%)
instructions in affected programs: 323664 -> 317894 (-1.78%)
helped: 1457
HURT: 0
helped stats (abs) min: 1 max: 12 x̄: 3.96 x̃: 3
helped stats (rel) min: 0.33% max: 11.49% x̄: 1.86% x̃: 1.44%
95% mean confidence interval for instructions value: -4.14 -3.78
95% mean confidence interval for instructions %-change: -1.93% -1.78%
Instructions are helped.

total cycles in shared programs: 187668016 -> 187657422 (<.01%)
cycles in affected programs: 14856234 -> 14845640 (-0.07%)
helped: 1372
HURT: 83
helped stats (abs) min: 2 max: 24 x̄: 7.92 x̃: 6
helped stats (rel) min: 0.02% max: 1.14% x̄: 0.12% x̃: 0.08%
HURT stats (abs)   min: 2 max: 14 x̄: 3.20 x̃: 2
HURT stats (rel)   min: 0.03% max: 0.60% x̄: 0.12% x̃: 0.12%
95% mean confidence interval for cycles value: -7.65 -6.91
95% mean confidence interval for cycles %-change: -0.11% -0.10%
Cycles are helped.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>