intel/vec4: Try immediate sources for dot products too

intel/vec4: Try immediate sources for dot products too

No changes on any Gen8 or later platform because those platforms do not
use the vec4 backend.

All Haswell and earlier platforms has similar results. (Haswell shown)
total instructions in shared programs: 13484467 -> 13484431 (<.01%)
instructions in affected programs: 8540 -> 8504 (-0.42%)
helped: 33
HURT: 0
helped stats (abs) min: 1 max: 2 x̄: 1.09 x̃: 1
helped stats (rel) min: 0.31% max: 1.53% x̄: 0.49% x̃: 0.35%
95% mean confidence interval for instructions value: -1.19 -0.99
95% mean confidence interval for instructions %-change: -0.60% -0.38%
Instructions are helped.

total cycles in shared programs: 376420572 -> 376420286 (<.01%)
cycles in affected programs: 56260 -> 55974 (-0.51%)
helped: 26
HURT: 5
helped stats (abs) min: 2 max: 204 x̄: 11.85 x̃: 2
helped stats (rel) min: 0.11% max: 3.08% x̄: 0.39% x̃: 0.13%
HURT stats (abs)   min: 2 max: 6 x̄: 4.40 x̃: 6
HURT stats (rel)   min: 0.03% max: 0.35% x̄: 0.24% x̃: 0.35%
95% mean confidence interval for cycles value: -22.91 4.45
95% mean confidence interval for cycles %-change: -0.56% -0.02%
Inconclusive result (value mean confidence interval includes 0).

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