[AArch64][SVE2] Asm: add unpredicated integer multiply instructions

Summary:
Add support for the following instructions:

  * MUL (indexed and unpredicated vectors forms)
  * SQDMULH (indexed and unpredicated vectors forms)
  * SQRDMULH (indexed and unpredicated vectors forms)
  * SMULH (unpredicated, predicated form added in SVE)
  * UMULH (unpredicated, predicated form added in SVE)
  * PMUL (unpredicated)

The specification can be found here:
https://developer.arm.com/docs/ddi0602/latest

Reviewed By: SjoerdMeijer, rovka

Differential Revision: https://reviews.llvm.org/D61902

llvm-svn: 360867