[AArch64][SVE] Folds VSELECT if the predicate is all active.

This adds the following changes:

* Fold: vselect(<all active predicate>, x, y) => x
* Extend isAllActivePredicate to take vscale_range into account, e.g.
  isAllActivePredicate(vl16) for nxv16i1 and vscale == 1 => true.
  isAllActivePredicate(vl32) for nxv16i1 and vscale == 2 => true.

Differential Revision: https://reviews.llvm.org/D118147