intel/fs: Eliminate dead code first

intel/fs: Eliminate dead code first

This simplifies the later patch "i965/fs: Generate better code for fsign
multiplied by a value".

shader-db results:

Broadwell and Skylake had similar results. (Skylake shown)
total cycles in shared programs: 372808735 -> 372809551 (<.01%)
cycles in affected programs: 1519520 -> 1520336 (0.05%)
helped: 243
HURT: 277
helped stats (abs) min: 1 max: 226 x̄: 34.05 x̃: 5
helped stats (rel) min: 0.01% max: 13.88% x̄: 1.46% x̃: 0.27%
HURT stats (abs)   min: 1 max: 1810 x̄: 32.82 x̃: 5
HURT stats (rel)   min: 0.01% max: 16.03% x̄: 1.56% x̃: 0.29%
95% mean confidence interval for cycles value: -7.18 10.32
95% mean confidence interval for cycles %-change: -0.17% 0.46%
Inconclusive result (value mean confidence interval includes 0).

Sandy Bridge, Haswell and Ivy Bridge had similar results. (Sandy Bridge shown)
total cycles in shared programs: 155091458 -> 155091109 (<.01%)
cycles in affected programs: 370797 -> 370448 (-0.09%)
helped: 24
HURT: 36
helped stats (abs) min: 1 max: 331 x̄: 103.17 x̃: 41
helped stats (rel) min: 0.02% max: 7.70% x̄: 2.07% x̃: 0.56%
HURT stats (abs)   min: 1 max: 291 x̄: 59.08 x̃: 10
HURT stats (rel)   min: 0.02% max: 5.29% x̄: 1.02% x̃: 0.15%
95% mean confidence interval for cycles value: -37.92 26.28
95% mean confidence interval for cycles %-change: -0.88% 0.45%
Inconclusive result (value mean confidence interval includes 0).

Iron Lake and GM45 had similar results. (GM45 shown)
total cycles in shared programs: 129133970 -> 129133978 (<.01%)
cycles in affected programs: 111966 -> 111974 (<.01%)
helped: 3
HURT: 1
helped stats (abs) min: 2 max: 4 x̄: 2.67 x̃: 2
helped stats (rel) min: <.01% max: <.01% x̄: <.01% x̃: <.01%
HURT stats (abs)   min: 16 max: 16 x̄: 16.00 x̃: 16
HURT stats (rel)   min: 0.07% max: 0.07% x̄: 0.07% x̃: 0.07%
95% mean confidence interval for cycles value: -12.93 16.93
95% mean confidence interval for cycles %-change: -0.05% 0.08%
Inconclusive result (value mean confidence interval includes 0).

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