[AArch64][SVE] Asm: Add SVE predicate register definitions and parsing support

Summary: Patch [1/4] in a series to add parsing of predicates and properly parse SVE ZIP1/ZIP2 instructions.

Reviewers: rengolin, kristof.beyls, fhahn, mcrosier, evandro, echristo, efriedma

Reviewed By: fhahn

Subscribers: aemerson, javed.absar, llvm-commits, tschuett

Differential Revision: https://reviews.llvm.org/D40360

llvm-svn: 320970