[AArch64][SVE] Asm: Support for gather LD1/LDFF1 (scalar + vector) load instructions.

[AArch64][SVE] Asm: Support for gather LD1/LDFF1 (scalar + vector) load instructions.

Patch [2/3] in series to add support for SVE's gather load instructions
that use scalar+vector addressing modes:
- Patch [1/3]: https://reviews.llvm.org/D45951
- Patch [2/3]: https://reviews.llvm.org/D46023
- Patch [3/3]: https://reviews.llvm.org/D45958

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

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D46023

llvm-svn: 330928