nir: Transform -fabs(a) < 0 to a != 0

nir: Transform -fabs(a) < 0 to a != 0

Unlike the much older -abs(a) >= 0.0 transformation, this is not
precise.  The behavior changes if a is NaN.

All Gen platforms had pretty similar results. (Skylake shown)
total instructions in shared programs: 14277216 -> 14277184 (<.01%)
instructions in affected programs: 2300 -> 2268 (-1.39%)
helped: 8
HURT: 0
helped stats (abs) min: 1 max: 8 x̄: 4.00 x̃: 3
helped stats (rel) min: 0.48% max: 15.15% x̄: 4.41% x̃: 1.01%
95% mean confidence interval for instructions value: -6.45 -1.55
95% mean confidence interval for instructions %-change: -9.96% 1.13%
Inconclusive result (%-change mean confidence interval includes 0).

total cycles in shared programs: 532577848 -> 532577990 (<.01%)
cycles in affected programs: 17486 -> 17628 (0.81%)
helped: 2
HURT: 5
helped stats (abs) min: 2 max: 6 x̄: 4.00 x̃: 4
helped stats (rel) min: 0.06% max: 1.81% x̄: 0.93% x̃: 0.93%
HURT stats (abs)   min: 6 max: 50 x̄: 30.00 x̃: 26
HURT stats (rel)   min: 0.55% max: 2.17% x̄: 1.19% x̃: 1.02%
95% mean confidence interval for cycles value: -1.06 41.63
95% mean confidence interval for cycles %-change: -0.58% 1.74%
Inconclusive result (value mean confidence interval includes 0).

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