nir/algebraic: Recognize open-coded copysign(1.0, a)

nir/algebraic: Recognize open-coded copysign(1.0, a)

All of the affected shaders are in Mad Max.  The inner part of the
pattern is itself an open-coded sign(a).  I tried using that as a
pattern, but the results were not good.  A bunch of shaders were helped
for instructions, but overall cycles, spill, and fills were hurt.

v2: Rebase on 1-bit Boolean changes.

v3: Fix order of copysign() parameters in comments and commit message.
Noticed by Matt.

All Gen7+ platforms had similar results. (Skylake shown)
total instructions in shared programs: 15282141 -> 15282073 (<.01%)
instructions in affected programs: 6106 -> 6038 (-1.11%)
helped: 17
HURT: 0
helped stats (abs) min: 4 max: 4 x̄: 4.00 x̃: 4
helped stats (rel) min: 1.02% max: 2.20% x̄: 1.15% x̃: 1.06%
95% mean confidence interval for instructions value: -4.00 -4.00
95% mean confidence interval for instructions %-change: -1.30% -1.00%
Instructions are helped.

total cycles in shared programs: 372597886 -> 372595954 (<.01%)
cycles in affected programs: 32701 -> 30769 (-5.91%)
helped: 17
HURT: 0
helped stats (abs) min: 6 max: 216 x̄: 113.65 x̃: 118
helped stats (rel) min: 0.40% max: 21.86% x̄: 6.20% x̃: 5.83%
95% mean confidence interval for cycles value: -152.84 -74.45
95% mean confidence interval for cycles %-change: -8.89% -3.51%
Cycles are helped.

No changes on any Gen6 or earlier platforms.

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