[IR][PatternMatch] Introduce m_NegatedPower2() matcher

Summary:
It is a good idea to do as much matching inside of `match()` as possible.
If some checking is done afterwards, and we don't fold because of it,
chances are we may have missed some commutative pattern.

Reviewers: spatel, craig.topper, RKSimon

Reviewed By: spatel, RKSimon

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64038

llvm-svn: 367017