nir/algebraic: some subtraction optimizations

The 2 fsub optimizations basically mirror the fadd optimizations after lowering.

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 %)