nir/algebraic: optimize (a < 0.0) ? -a : a -> fabs(a)

nir/algebraic: optimize (a < 0.0) ? -a : a -> fabs(a)

Totals from affected shaders: (VEGA)
SGPRS: 13920 -> 13920 (0.00 %)
VGPRS: 10252 -> 10252 (0.00 %)
Spilled SGPRs: 62 -> 62 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 587648 -> 587224 (-0.07 %) bytes
LDS: 5 -> 5 (0.00 %) blocks
Max Waves: 1489 -> 1489 (0.00 %)

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4830>