[AArch64][AsmParser] Unify code for parsing Neon/SVE vectors.

[AArch64][AsmParser] Unify code for parsing Neon/SVE vectors.

Summary:
Merged 'tryMatchVectorRegister' (specific to Neon) and
'tryParseSVERegister' into a single 'tryParseVectorRegister' function, and
created a generic 'parseVectorKind()' function that returns the #Elements
and ElementWidth of a vector suffix. This reduces the duplication of
this functionality between two the vector implementations.

This is patch [1/6] in a series to add assembler/disassembler support for
SVE's contiguous ST1 (scalar+imm) instructions.

Reviewers: fhahn, rengolin, javed.absar, huntergr, SjoerdMeijer, t.p.northover, echristo, evandro

Reviewed By: fhahn

Subscribers: tschuett, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D45427

llvm-svn: 329782