The patch refactors unroll pass.

Summary:
Unroll factor (Count) calculations moved to a new function.
Early exits on pragma and "-unroll-count" defined factor added.
New type of unrolling "Force" introduced (previously used implicitly).
New unroll preference "AllowRemainder" introduced and set "true" by default.
(should be set to false for architectures that suffers from it).

Reviewers: hfinkel, mzolotukhin, zzheng

Differential Revision: http://reviews.llvm.org/D19553

From: Evgeny Stupachenko <evstupac@gmail.com>
llvm-svn: 271071