Initializing NVPTX backend within Polly

Summary:
The NVPTX backend is now initialised within Polly. A language front-end need not be modified to initialise the backend, just for Polly.

Reviewers: Meinersbur, grosser

Reviewed By: Meinersbur

Subscribers: vchuravy, mgorny

Tags: #polly

Differential Revision: https://reviews.llvm.org/D31859

llvm-svn: 306649