intel/vec4: Try to emit immediate sources for MOV

intel/vec4: Try to emit immediate sources for MOV

Per the comment in vec4_visitor::nir_emit_load_const, further
improvement is possible in this area.  That case would be more
complicated as I think we'd want to check that all users of the
nir_load_const_instr result intended to use the value as float.

No shader-db changes on any Gen8+ platform as these platforms do not use
the vec4 backend.

v2: Massive rebase on eeebeb211f1 ("intel/vec4: Try emitting non-scalar
immediates").  This commit is about twice as helpful since b04beaf41d2
("intel/vec4: Try both sources as candidates for being immediates").

Haswell and Ivy Bridge had similar results. (Haswell shown)
total instructions in shared programs: 13478598 -> 13474068 (-0.03%)
instructions in affected programs: 589452 -> 584922 (-0.77%)
helped: 2773
HURT: 0
helped stats (abs) min: 1 max: 7 x̄: 1.63 x̃: 1
helped stats (rel) min: 0.16% max: 5.66% x̄: 0.96% x̃: 0.83%
95% mean confidence interval for instructions value: -1.67 -1.60
95% mean confidence interval for instructions %-change: -0.98% -0.94%
Instructions are helped.

total cycles in shared programs: 376386916 -> 376369392 (<.01%)
cycles in affected programs: 16871628 -> 16854104 (-0.10%)
helped: 2293
HURT: 523
helped stats (abs) min: 2 max: 812 x̄: 13.80 x̃: 2
helped stats (rel) min: <.01% max: 10.18% x̄: 1.02% x̃: 0.36%
HURT stats (abs)   min: 2 max: 316 x̄: 26.99 x̃: 14
HURT stats (rel)   min: <.01% max: 19.34% x̄: 2.15% x̃: 1.43%
95% mean confidence interval for cycles value: -7.87 -4.58
95% mean confidence interval for cycles %-change: -0.52% -0.34%
Cycles are helped.

Sandy Bridge
total instructions in shared programs: 10860328 -> 10857675 (-0.02%)
instructions in affected programs: 335907 -> 333254 (-0.79%)
helped: 1639
HURT: 0
helped stats (abs) min: 1 max: 5 x̄: 1.62 x̃: 1
helped stats (rel) min: 0.10% max: 5.26% x̄: 0.86% x̃: 0.70%
95% mean confidence interval for instructions value: -1.67 -1.57
95% mean confidence interval for instructions %-change: -0.89% -0.84%
Instructions are helped.

total cycles in shared programs: 153942720 -> 153934120 (<.01%)
cycles in affected programs: 5604818 -> 5596218 (-0.15%)
helped: 1494
HURT: 97
helped stats (abs) min: 2 max: 256 x̄: 7.84 x̃: 2
helped stats (rel) min: 0.01% max: 6.62% x̄: 0.35% x̃: 0.18%
HURT stats (abs)   min: 2 max: 160 x̄: 32.02 x̃: 20
HURT stats (rel)   min: 0.02% max: 3.37% x̄: 0.88% x̃: 0.56%
95% mean confidence interval for cycles value: -6.45 -4.36
95% mean confidence interval for cycles %-change: -0.32% -0.23%
Cycles are helped.

Iron Lake and GM45 had similar results. (Iron Lake shown)
total instructions in shared programs: 8139378 -> 8137267 (-0.03%)
instructions in affected programs: 265616 -> 263505 (-0.79%)
helped: 1148
HURT: 0
helped stats (abs) min: 1 max: 5 x̄: 1.84 x̃: 1
helped stats (rel) min: 0.22% max: 4.76% x̄: 0.87% x̃: 0.62%
95% mean confidence interval for instructions value: -1.90 -1.78
95% mean confidence interval for instructions %-change: -0.90% -0.83%
Instructions are helped.

total cycles in shared programs: 188541756 -> 188537540 (<.01%)
cycles in affected programs: 9807004 -> 9802788 (-0.04%)
helped: 1143
HURT: 4
helped stats (abs) min: 2 max: 10 x̄: 3.70 x̃: 2
helped stats (rel) min: <.01% max: 3.01% x̄: 0.13% x̃: 0.06%
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 0.18% max: 0.18% x̄: 0.18% x̃: 0.18%
95% mean confidence interval for cycles value: -3.80 -3.55
95% mean confidence interval for cycles %-change: -0.14% -0.12%
Cycles are helped.

Reviewed-by: Matt Turner <mattst88@gmail.com>