Refactor the build{Module|Function}SimplificationPipeline to expose optimization phase.

Summary: This is in preparation of https://reviews.llvm.org/D36052

Reviewers: chandlerc, davidxl, tejohnson

Reviewed By: chandlerc

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D36053

llvm-svn: 309500