[SCEV] Try to prove predicates by splitting them

Summary:
This change teaches SCEV that to prove `A u< B` it is sufficient to
prove each of these facts individually:

 - B >= 0
 - A s< B
 - A >= 0

In practice, SCEV sometimes finds it easier to prove these facts
individually than to prove `A u< B` as one atomic step.

Reviewers: reames, atrick, nlewycky, hfinkel

Subscribers: sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D13042

llvm-svn: 249168