[Polly][NewPM] Port CodeGen to the new PM

Summary: To move CG to the new PM I outlined the various helper that were previously members of the CG class into free static functions. The CG class itself I moved into a header, which is required because we need to include it in `RegisterPasses` eventually.

Reviewers: grosser, Meinersbur

Reviewed By: grosser

Subscribers: pollydev, llvm-commits, sanjoy

Tags: #polly

Differential Revision: https://reviews.llvm.org/D33423

llvm-svn: 303624