[AArch64][SVE] Asm: Add SVE (Z) Register definitions and parsing support

Patch [3/5] in a series to add assembler/disassembler support for AArch64 SVE unpredicated ADD/SUB instructions.

To summarise, this patch adds:

 * SVE register definitions
 * Methods to parse SVE register operands
 * Methods to print SVE register operands
 * RegKind SVEDataVector to distinguish it from other data types like scalar register or Neon vector.
 * k_SVEDataRegister and SVEDataRegOp to describe SVE registers (which will be extended by further patches with e.g. ElementWidth and the shift-extend type).


Patch by Sander De Smalen.

Reviewed by: rengolin

Differential Revision: https://reviews.llvm.org/D39089

llvm-svn: 317590