nir/algebraic: optimize fmul(x, bcsel(c, -1.0, 1.0)) -> bcsel(c, -x, x)

nir/algebraic: optimize fmul(x, bcsel(c, -1.0, 1.0)) -> bcsel(c, -x, x)

Totals from affected shaders: (VEGA)
SGPRS: 545712 -> 545712 (0.00 %)
VGPRS: 413092 -> 413116 (0.01 %)
Spilled SGPRs: 10616 -> 10616 (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: 37031684 -> 36984248 (-0.13 %) bytes
LDS: 427 -> 427 (0.00 %) blocks
Max Waves: 54350 -> 54340 (-0.02 %)

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>