[AArch64][SVE] Asm: Support for structured ST2, ST3 and ST4 (scalar+imm) store instructions.

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

Reviewed By: rengolin

Subscribers: tschuett, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D45681

llvm-svn: 330565