[LV][NFC] Factor out calculation of "best" estimated trip count.

This is just small refactoring to minimize changes in upcoming patch.
In the next path I'm going to introduce changes into heuristic for vectorization of "tiny trip count" loops.

Patch by Evgeniy Brevnov <evgueni.brevnov@gmail.com>

Reviewers: hsaito, Ayal, fhahn, reames

Reviewed By: hsaito

Differential Revision: https://reviews.llvm.org/D67690

llvm-svn: 374338