[AArch64][SVE] Asm: Support for saturating INC/DEC (64bit scalar) instructions.

Summary:
The variants added by this patch are:
- SQINC  (signed increment)
- UQINC  (unsigned increment)
- SQDEC  (signed decrement)
- UQDEC  (unsigned decrement)

For example:
  uqincw  x0, all, mul #4

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

Differential Revision: https://reviews.llvm.org/D47715

llvm-svn: 334948