Introduce a hybrid target to generate code for either the GPU or CPU

Summary:
Introduce a "hybrid" `-polly-target` option to optimise code for either the GPU or CPU.

When this target is selected, PPCGCodeGeneration will attempt first to optimise a Scop. If the Scop isn't modified, it is then sent to the passes that form the CPU pipeline, i.e. IslScheduleOptimizerPass, IslAstInfoWrapperPass and CodeGeneration.

In case the Scop is modified, it is marked to be skipped by the subsequent CPU optimisation passes.

Reviewers: grosser, Meinersbur, bollu

Reviewed By: grosser

Subscribers: kbarton, nemanjai, pollydev

Tags: #polly

Differential Revision: https://reviews.llvm.org/D34054

llvm-svn: 306863