[AArch64][SVE] Asm: Add MOVPRFX instructions.

This patch adds predicated and unpredicated MOVPRFX instructions, which
can be prepended to SVE instructions that are destructive on their first
source operand, to make them a constructive operation, e.g.

  add z1.s, p0/m, z1.s, z2.s        <=> z1 = z1 + z2

can be made constructive:

  movprfx z0, z1
  add z0.s, p0/m, z0.s, z2.s        <=> z0 = z1 + z2

The predicated MOVPRFX instruction can additionally be used to zero
inactive elements, e.g.

  movprfx z0.s, p0/z, z1.s
  add z0.s, p0/m, z0.s, z2.s

Not all instructions can be prefixed with the MOVPRFX instruction
which is why this patch also adds a mechanism to validate prefixed
instructions. The exact rules when a MOVPRFX applies is detailed in
the SVE supplement of the Architectural Reference Manual.

This is patch [1/2] 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/D49592

llvm-svn: 338258