[IR][Patternmatch] Add m_SpecificInt_ULT() predicate

Summary:
Match an integer or vector with every element unsigned less than the
Threshold. For vectors, this includes constants with undefined elements.

FIXME: is it worth generalizing this to simply take ICmpInst::Predicate?

Reviewers: craig.topper, spatel, nikic

Reviewed By: spatel

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63811

llvm-svn: 364711