radv/aco: Don't lower subtractions

radv/aco: Don't lower subtractions

40228 shaders in 20236 tests
Totals:
SGPRS: 2045512 -> 2046496 (0.05 %)
VGPRS: 1430856 -> 1430464 (-0.03 %)
Spilled SGPRs: 1077 -> 1077 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 10348 -> 10348 (0.00 %) dwords per thread
Code Size: 77202840 -> 77151832 (-0.07 %) bytes
LDS: 863 -> 863 (0.00 %) blocks
Max Waves: 260729 -> 260754 (0.01 %)

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>