[AArch64][SVE] Asm: Add AsmOperand classes for SVE gather/scatter addressing modes.

This patch adds parsing support for 'vector + shift/extend' and
corresponding asm operand classes, needed for implementing SVE's
gather/scatter addressing modes.

The added combinations of vector (ZPR) and Shift/Extend are:

Unscaled:
  ZPR64ExtLSL8:           signed 64-bit offsets  (z0.d)
  ZPR32ExtUXTW8:        unsigned 32-bit offsets  (z0.s, uxtw)
  ZPR32ExtSXTW8:          signed 32-bit offsets  (z0.s, sxtw)

Unpacked and unscaled:
  ZPR64ExtUXTW8:        unsigned 32-bit offsets  (z0.d, uxtw)
  ZPR64ExtSXTW8:          signed 32-bit offsets  (z0.d, sxtw)

Unpacked and scaled:
  ZPR64ExtUXTW<scale>:  unsigned 32-bit offsets  (z0.d, uxtw #<shift>)
  ZPR64ExtSXTW<scale>:    signed 32-bit offsets  (z0.d, sxtw #<shift>)

Scaled:
  ZPR32ExtUXTW<scale>:  unsigned 32-bit offsets  (z0.s, uxtw #<shift>)
  ZPR32ExtSXTW<scale>:    signed 32-bit offsets  (z0.s, sxtw #<shift>)
  ZPR64ExtLSL<scale>:   unsigned 64-bit offsets  (z0.d,  lsl #<shift>)
  ZPR64ExtLSL<scale>:     signed 64-bit offsets  (z0.d,  lsl #<shift>)


Patch [1/3] 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/D45951

llvm-svn: 330805