Update loop unroller cost model to make sure debug info does not affect optimization decisions.

Summary: Debug info should *not* affect optimization decisions. This patch updates loop unroller cost model to make it not affected by debug info.

Reviewers: davidxl, mzolotukhin

Subscribers: haicheng, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D25098

llvm-svn: 282894