nir_to_tgsi: Add a flag for lowering fabs, and use it in r300/i915.

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%)

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14938>