You can encode a 12-bit immediate in an ADD instruction on ARM64. If the
negated constant fits in this range, we can exploit this to avoid
materializing the immediate.
This approach saves an instruction if it does not need to be
materialized in a register afterwards. Otherwise, we just materialize
it later and the total number of instructions stays the same.
Before:
0x12800019 mov w25, #-0x1 ; =-1
0x93407f5b sxtw x27, w26
0xcb39c37b sub x27, x27, w25, sxtw
After:
0x93407f5b sxtw x27, w26
0x9100077b add x27, x27, #0x1