nir/flrp: Lower flrp(±1, b, c) and flrp(a, ±1, c) differently

nir/flrp: Lower flrp(±1, b, c) and flrp(a, ±1, c) differently

No changes on any other Intel platforms.

v2: Rebase on 424372e5dd5 ("nir: Use the flrp lowering pass instead of
nir_opt_algebraic")

Iron Lake and GM45 had similar results. (Iron Lake shown)
total instructions in shared programs: 8189888 -> 8153912 (-0.44%)
instructions in affected programs: 1199037 -> 1163061 (-3.00%)
helped: 4124
HURT: 10
helped stats (abs) min: 1 max: 40 x̄: 8.73 x̃: 9
helped stats (rel) min: 0.20% max: 86.96% x̄: 4.96% x̃: 3.02%
HURT stats (abs)   min: 1 max: 2 x̄: 1.20 x̃: 1
HURT stats (rel)   min: 1.06% max: 3.92% x̄: 1.62% x̃: 1.06%
95% mean confidence interval for instructions value: -8.84 -8.56
95% mean confidence interval for instructions %-change: -5.12% -4.77%
Instructions are helped.

total cycles in shared programs: 188606710 -> 188426964 (-0.10%)
cycles in affected programs: 27505596 -> 27325850 (-0.65%)
helped: 4026
HURT: 77
helped stats (abs) min: 2 max: 646 x̄: 44.99 x̃: 46
helped stats (rel) min: <.01% max: 94.58% x̄: 2.35% x̃: 0.85%
HURT stats (abs)   min: 2 max: 376 x̄: 17.79 x̃: 6
HURT stats (rel)   min: <.01% max: 2.60% x̄: 0.22% x̃: 0.04%
95% mean confidence interval for cycles value: -44.75 -42.87
95% mean confidence interval for cycles %-change: -2.44% -2.17%
Cycles are helped.

LOST:   3
GAINED: 35

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