nir_to_tgsi: Add a flag for lowering fabs, and use it in r300/i915.
Saves instructions if the same fabs value is used multiple times.
i915g:
total instructions in shared programs: 397005 -> 396525 (-0.12%)
instructions in affected programs: 11061 -> 10581 (-4.34%)
LOST: 0
GAINED: 22
r300 (not r500):
total instructions in shared programs: 180286 -> 179767 (-0.29%)
instructions in affected programs: 27102 -> 26583 (-1.91%)
total temps in shared programs: 29692 -> 29638 (-0.18%)
temps in affected programs: 356 -> 302 (-15.17%)