nir: Simplify 0 < fabs(a)

nir: Simplify 0 < fabs(a)

Sandy Bridge / Ivy Bridge / Haswell
total instructions in shared programs: 8462180 -> 8462174 (-0.00%)
instructions in affected programs: 564 -> 558 (-1.06%)
helped: 6
HURT: 0

total cycles in shared programs: 117542462 -> 117542276 (-0.00%)
cycles in affected programs: 9768 -> 9582 (-1.90%)
helped: 12
HURT: 0

Broadwell / Skylake
total instructions in shared programs: 8980833 -> 8980826 (-0.00%)
instructions in affected programs: 626 -> 619 (-1.12%)
helped: 7
HURT: 0

total cycles in shared programs: 70077900 -> 70077714 (-0.00%)
cycles in affected programs: 9378 -> 9192 (-1.98%)
helped: 12
HURT: 0

G45 and Ironlake showed no change.

v2: Modify the comments to look more like a proof.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>