Minor unroll pass refacoring.

Summary:
Unrolled Loop Size calculations moved to a function.
Constant representing number of optimized instructions
 when "back edge" becomes "fall through" replaced with
 variable.
Some comments added.

Reviewers: mzolotukhin

Differential Revision: http://reviews.llvm.org/D21719

From: Evgeny Stupachenko <evstupac@gmail.com>
llvm-svn: 286389