[AArch64][SVE] Fold away SETCC if original input was predicate vector.

This adds the following two folds:

Fold 1:
   setcc_merge_zero(
       all_active, extend(nxvNi1 ...), != splat(0))
  -> nxvNi1 ...

Fold 2:
   setcc_merge_zero(
       pred, extend(nxvNi1 ...), != splat(0))
  -> nxvNi1 and(pred, ...)

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D119334