[AArch64][SVE] Asm: Add support for SVE INDEX instructions.

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

Reviewed By: rengolin, fhahn

Subscribers: tschuett, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D45370

llvm-svn: 329674