[NewPM] Update pass registration for the LLVM plugin interface

Summary:
As of rL329273, LLVM has a mechanism to load new-pm plugins in opt. Use
this API in Polly.

Reviewers: grosser, Meinersbur, bollu

Reviewed By: grosser, Meinersbur

Subscribers: lksbhm, bollu, pollydev, llvm-commits

Differential Revision: https://reviews.llvm.org/D45484

llvm-svn: 330181