Refactor LICM pass in preparation for LoopSink pass.

Summary: LoopSink pass uses some common function in LICM. This patch refactor the LICM code to make it usable by LoopSink pass (https://reviews.llvm.org/D22778).

Reviewers: davidxl, danielcdh, hfinkel, chandlerc

Subscribers: hfinkel, llvm-commits

Differential Revision: https://reviews.llvm.org/D24168

llvm-svn: 283134