[AArch64][SVE] Asm: Vector Unpack Low/High instructions.

This patch adds support for the following unpack instructions:
  
- PUNPKLO, PUNPKHI   Unpack elements from low/high half and
                     place into elements of twice their size.

  e.g. punpklo p0.h, p0.b

- UUNPKLO, UUNPKHI   Unpack elements from low/high half and 
  SUNPKLO, SUNPKHI   place into elements of twice their size
                     after zero- or sign-extending the values.

  e.g. uunpklo z0.h, z0.b

llvm-svn: 336982