[AArch64][AsmParser] Split index parsing from vector list.

Summary:
Place parsing of a vector index into a separate function to reduce
duplication, since the code is duplicated in both the parsing of a
Neon vector register operand and a Neon vector list.

This is patch [2/6] 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: rengolin

Subscribers: kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D45428

llvm-svn: 329809