[LoopDeletion] Move ICmpInst handling to getValueOnFirstIteration()

As noticed in https://reviews.llvm.org/D105688, it would be great to move
handling of ICmpInst which was in canProveExitOnFirstIteration() to
getValueOnFirstIteration().

Patch by Dmitry Makogon!

Differential Revision: https://reviews.llvm.org/D108978
Reviewed By: reames