nir/algebraic: add optimizations for fsign/isign

nir/algebraic: add optimizations for fsign/isign

This just reverts fsign/isign lowering.

Totals from affected shaders:
SGPRS: 257496 -> 256672 (-0.32 %)
VGPRS: 181800 -> 178864 (-1.61 %)
Spilled SGPRs: 105 -> 105 (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: 11355852 -> 11141840 (-1.88 %) bytes
LDS: 3789 -> 3789 (0.00 %) blocks
Max Waves: 30453 -> 30951 (1.64 %)

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>