[IR][PatternMatch] introduce m_Unless() matcher

Summary:
I don't think it already exists? I don't see it at least.
It is important to have it because else we'll do some checks after `match()`,
and that may result in missed folds in commutative nodes.

Reviewers: spatel, craig.topper, RKSimon, majnemer

Reviewed By: spatel

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64037

llvm-svn: 367016