We were not accountint for small immediates in the B mux so the scheduler
was interpreting these are regular register file accesses, which could
lead to additional (incorrect) write-read dependencies.

Shader-db changes:

total instructions in shared programs: 9163664 -> 9137263 (-0.29%)
instructions in affected programs: 3931035 -> 3904634 (-0.67%)
helped: 12457
HURT: 2563

total max-temps in shared programs: 1325787 -> 1325597 (-0.01%)
max-temps in affected programs: 5746 -> 5556 (-3.31%)
helped: 186
HURT: 16
helped stats (abs) min: 1 max: 4 x̄: 1.12 x̃: 1
helped stats (rel) min: 1.45% max: 22.22% x̄: 4.42% x̃: 3.28%
HURT stats (abs)   min: 1 max: 3 x̄: 1.12 x̃: 1
HURT stats (rel)   min: 2.86% max: 10.00% x̄: 5.76% x̃: 5.88%
95% mean confidence interval for max-temps value: -1.04 -0.84
95% mean confidence interval for max-temps %-change: -4.16% -3.07%
Max-temps are helped.

Reviewed-by: Eric Anholt <eric@anholt.net>