[ARM] Turn pred_cast(xor(x, -1)) into xor(pred_cast(x), -1)

This folds a not (an xor -1) though a predicate_cast, so that it can be
turned into a VPNOT and potentially be folded away as an else predicate
inside a VPT block.

Differential Revision: https://reviews.llvm.org/D92235