[AArch64][SVE] Asm: Support for WHILE(LE|LO|LS|LT) instructions.

The WHILE instructions generate a predicate that is true while the 
comparison of the first scalar operand (incremented for each predicate
element) with the second scalar operand is true and false thereafter.

  WHILELE  While incrementing signed scalar less than or equal to scalar
  WHILELO  While incrementing unsigned scalar lower than scalar
  WHILELS  While incrementing unsigned scalar lower than or same as scalar
  WHILELT  While incrementing signed scalar less than scalar

e.g.

  whilele  p0.s, x0, x1

  generates predicate p0 (for 32bit elements) by incrementing
  (signed) x0 and comparing that vector to splat(x1).

llvm-svn: 338211