[AArch64][AsmParser] Unify 'addVectorListOperands' functions.

Summary:
Merged 'addVectorList64Operands' and 'addVectorList128Operands' into a
generic 'addVectorListOperands', which can be easily extended to work
for SVE vectors.

This is patch [4/6] 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/D45430

llvm-svn: 329909