nir: Transform -fabs(a) >= 0 to a == 0

nir: Transform -fabs(a) >= 0 to a == 0

All Gen platforms had pretty similar results. (Skylake shown)
total instructions in shared programs: 14276964 -> 14276961 (<.01%)
instructions in affected programs: 411 -> 408 (-0.73%)
helped: 3
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.47% max: 1.96% x̄: 1.04% x̃: 0.68%

total cycles in shared programs: 532577062 -> 532577068 (<.01%)
cycles in affected programs: 1093 -> 1099 (0.55%)
helped: 1
HURT: 1
helped stats (abs) min: 16 max: 16 x̄: 16.00 x̃: 16
helped stats (rel) min: 7.77% max: 7.77% x̄: 7.77% x̃: 7.77%
HURT stats (abs)   min: 22 max: 22 x̄: 22.00 x̃: 22
HURT stats (rel)   min: 2.48% max: 2.48% x̄: 2.48% x̃: 2.48%

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>