[AArch64][SVE] Asm: Add parsing of merging/zeroing suffix for SVE predicate vector operands

Summary:
Parsing of the '/m' (merging) or '/z' (zeroing) suffix of a predicate operand.

Patch [2/3] in a series to add predicated ADD/SUB instructions for SVE.

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

Reviewed By: fhahn

Subscribers: t.p.northover, MatzeB, aemerson, javed.absar, tschuett, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D41442

llvm-svn: 322070