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

Increment/decrement vector by multiple of predicate constraint
element count.

The variants added by this patch are:
 - INCH, INCW, INC 

and (saturating):
 - SQINCH, SQINCW, SQINCD
 - UQINCH, UQINCW, UQINCW
 - SQDECH, SQINCW, SQINCD
 - UQDECH, UQINCW, UQINCW

For example:
  incw z0.s, all, mul #4

llvm-svn: 336090