[AArch64][SVE] Asm: Support for integer MUL instructions.

This patch adds the following instructions:
  MUL   - multiply vectors, e.g.
    mul z0.h, p0/m, z0.h, z1.h
        - multiply with immediate, e.g.
    mul z0.h, z0.h, #127

  SMULH - signed multiply returning high half, e.g.
    smulh z0.h, p0/m, z0.h, z1.h

  UMULH - unsigned multiply returning high half, e.g.
    umulh z0.h, p0/m, z0.h, z1.h

llvm-svn: 337358