[AArch64][SVE] Asm: Support for predicated unary operations.

The patch includes support for the following instructions:

       ABS z0.h, p0/m, z0.h
       NEG z0.h, p0/m, z0.h

  (S|U)XTB z0.h, p0/m, z0.h
  (S|U)XTB z0.s, p0/m, z0.s
  (S|U)XTB z0.d, p0/m, z0.d

  (S|U)XTH z0.s, p0/m, z0.s
  (S|U)XTH z0.d, p0/m, z0.d

  (S|U)XTW z0.d, p0/m, z0.d

llvm-svn: 336204