[AArch64][SVE] Asm: Support for saturing ADD/SUB instructions.

The variants added are:
    signed Saturating ADD/SUB (immediate)  e.g. sqadd z0.h, z0.h, #42
  unsigned Saturating ADD/SUB (immediate)  e.g. uqadd z0.h, z0.h, #42
    signed Saturating ADD/SUB (vectors)    e.g. sqadd z0.h, z0.h, z1.h
  unsigned Saturating ADD/SUB (vectors)    e.g. uqadd z0.h, z0.h, z1.h

llvm-svn: 336186