[ValueTracking] Add a framework for encoding implication rules

Summary:
This change adds a framework for adding more smarts to
`isImpliedCondition` around inequalities.  Informally,
`isImpliedCondition` will now try to prove "A < B ==> C < D" by proving
"C <= A && B <= D", since then it follows "C <= A < B <= D".

While this change is in principle NFC, I could not think of a way to not
handle cases like "i +_nsw 1 < L ==> i < L +_nsw 1" (that ValueTracking
did not handle before) while keeping the change understandable.  I've
added tests for these cases.

Reviewers: reames, majnemer, hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14368

llvm-svn: 252331