[ScalarEvolution] Handling for ICmp occuring in the evolution chain.

Summary:
 If a compare instruction is same or inverse of the compare in the
 branch of the loop latch, then return a constant evolution node.
 Currently scope of evaluation is limited to SCEV computation for
 PHI nodes.

 This shall facilitate computations of loop exit counts in cases
 where compare appears in the evolution chain of induction variables.

 Will fix PR 34538
Reviewers: sanjoy, hfinkel, junryoungju

Reviewed By: junryoungju

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D38494

llvm-svn: 316054