Ignore debug info when making optimization decisions in SimplifyCFG.

Summary: Debug info should *not* affect code generation. This patch properly handles debug info to make sure the generated code are the same with or without debug info.

Reviewers: davidxl, mzolotukhin, jmolloy

Subscribers: aprantl, llvm-commits

Differential Revision: https://reviews.llvm.org/D25286

llvm-svn: 284415