[Polly][PM] Improve invalidation in the Scop-Pipeline

Summary:
During code generation for a Scop we modify the IR of a function.
While this shouldn't affect a Scop in the formal sense, the implementation
caches various information about the IR such as SCEV expressions for bounds or
parameters. This cached information needs to be updated or invalidated. To this
end, SPMUpdater allows passes to report when they've invalidated a Scop to the
PassManager, which will then flush and recompute all Scops. This in turn
invalidates all iterators, so references to Scops shouldn't be held.

Reviewers: grosser, Meinersbur, bollu

Reviewed By: grosser

Subscribers: llvm-commits, pollydev

Differential Revision: https://reviews.llvm.org/D36524

llvm-svn: 310551