[AArch64][SVE] Asm: Support for reversed subtract (SUBR) instruction.
This patch adds both a vector and an immediate form, e.g.
- Vector form:
subr z0.h, p0/m, z0.h, z1.h
subtract active elements of z0 from z1, and store the result in z0.
- Immediate form:
subr z0.h, z0.h, #255
subtract elements of z0, and store the result in z0.