[AArch64][SVE] Asm: Support for structured LD2 (scalar+imm) load instructions.

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

Reviewed By: rengolin

Subscribers: tschuett, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D45622

llvm-svn: 330108