[ARM] Common inverse constant predicates to VPNOT

This scans through blocks looking for constants used as predicates in
MVE instructions. When two constants are found which are the inverse of
one another, the second can be replaced by a VPNOT of the first,
potentially allowing that not to be folded away into an else predicate
of a vpt block.

Differential Revision: https://reviews.llvm.org/D92470