Reimplement heuristic for estimating complete-unroll optimization effects.

Summary:
This patch reimplements heuristic that tries to estimate optimization beneftis
from complete loop unrolling.

In this patch I kept the minimal changes - e.g. I removed code handling
branches and folding compares. That's a promising area, but now there
are too many questions to discuss before we can enable it.

Test Plan: Tests are included in the patch.

Reviewers: hfinkel, chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8816

llvm-svn: 237156