[SCEV] 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.
 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: sanjoy, junryoungju

Subscribers: javed.absar, llvm-commits

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

llvm-svn: 318050