[AArch64][SVE] Asm: Predicate patterns

Summary:
This patch adds support for parsing/printing of named or unnamed
patterns that are used in SVE's PTRUE instruction, amongst others.

The pattern can be specified as a named pattern to initialize the predicate
vector or it can be specified as an immediate in the range 0-31.

Reviewers: fhahn, rengolin, evandro, mcrosier, t.p.northover

Reviewed By: fhahn

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

Differential Revision: https://reviews.llvm.org/D41818

llvm-svn: 323098