[AArch64][SVE] Asm: Enable instructions to be prefixed.

This patch enables instructions that are destructive on their
destination- and first source operand, to be prefixed with a
MOVPRFX instruction.

This patch also adds a variety of tests:

- positive tests for all instructions and forms that accept a
  movprfx for either or both predicated and unpredicated forms.

- negative tests for all instructions and forms that do not accept
  an unpredicated or predicated movprfx.

- negative tests for the diagnostics that get emitted when a MOVPRFX
  instruction is used incorrectly.

This is patch [2/2] in a series to add MOVPRFX instructions:
- Patch [1/2]: https://reviews.llvm.org/D49592
- Patch [2/2]: https://reviews.llvm.org/D49593

Reviewers: rengolin, SjoerdMeijer, samparker, fhahn, javed.absar

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D49593

llvm-svn: 338261