[AArch64][SVE] Asm: Support for EXT instruction.

This patch adds an instruction that allows extracting
a vector from a pair of vectors, given an immediate index
that describes the element position to extract from.

The instruction has the following assembly:
  ext z0.b, z0.b, z1.b, #imm

where #imm is an immediate between 0 and 255.

llvm-svn: 337251