[AArch64][SVE] Asm: Support for saturating INC/DEC (32bit scalar) instructions.

The variants added by this patch are:
- SQINC     signed increment, e.g. sqinc x0, w0, all, mul #4
- SQDEC     signed decrement, e.g. sqdec x0, w0, all, mul #4
- UQINC   unsigned increment, e.g. uqinc w0, all, mul #4
- UQDEC   unsigned decrement, e.g. uqdec w0, all, mul #4
 
This patch includes asmparser changes to parse a GPR64 as a GPR32 in
order to satisfy the constraint check:
  x0 == GPR64(w0)
in:
  sqinc x0, w0, all, mul #4
         ^___^ (must match)

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

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D47716

llvm-svn: 334980