[SCEV] Reapply 'Teach isLoopBackedgeGuardedByCond to exploit trip counts'

[SCEV] Reapply 'Teach isLoopBackedgeGuardedByCond to exploit trip counts'

Summary:
If the trip count of a specific backedge is `N`, then we know that
backedge is effectively guarded by the condition `{0,+,1} u< N`.  This
change teaches SCEV to use this condition to prove things in
`isLoopBackedgeGuardedByCond`.

Depends on D12948
Depends on D12949

The original checkin, r248608 had to be backed out due to an issue with
a ObjCXX unit test.  That issue is now fixed, so re-landing.

Reviewers: atrick, reames, majnemer, hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12950

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248638 91177308-0d34-0410-b5e6-96231b3b80d8