nir/algebraic: some subtraction optimizations

nir/algebraic: some subtraction optimizations

Changes with RADV/ACO:
Totals from affected shaders:
SGPRS: 444087 -> 455543 (2.58 %)
VGPRS: 436468 -> 436768 (0.07 %)
Spilled SGPRs: 0 -> 0 (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: 13448928 -> 13353520 (-0.71 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 68060 -> 67979 (-0.12 %)
Wait states: 0 -> 0 (0.00 %)

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>