[SCEV] Exploit A < B => (A+K) < (B+K) when possible

Summary:

This change teaches SCEV's `isImpliedCond` two new identities:

  A u< B u< -C          =>  (A + C) u< (B + C)
  A s< B s< INT_MIN - C =>  (A + C) s< (B + C)

While these are useful on their own, they're really intended to support
D12950.

Reviewers: atrick, reames, majnemer, nlewycky, hfinkel

Subscribers: aadg, sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D12948

llvm-svn: 248606