[AArch64][SVE] Asm: Support for contiguous ST1 (scalar+imm) store instructions.

Summary:
Added instructions for contiguous stores, ST1, with scalar+imm addressing
modes and corresponding tests. The patch also adds parsing of
'mul vl' as needed for the VL-scaled immediate.

This is patch [6/6] 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: tschuett, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D45432

llvm-svn: 330014