[AArch64][SVE] Asm: Support for contiguous LD1 (scalar+scalar) load instructions.

This is patch [4/4] in a series to add assembler/disassembler support for
SVE's contiguous LD1 (scalar+scalar) instructions:
- Patch [1/4]: https://reviews.llvm.org/D45687
- Patch [2/4]: https://reviews.llvm.org/D45688
- Patch [3/4]: https://reviews.llvm.org/D45689
- Patch [4/4]: https://reviews.llvm.org/D45690

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

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D45690

llvm-svn: 330423