[PM] Don't walk the AM's ResultsList if nothing was invalidated.

Summary:
Previously in AnalysisManager::invalidate(), we would walk the full
ResultsList even if we knew that nothing was invalidated.

Reviewers: chandlerc

Subscribers: silvas, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D27371

llvm-svn: 288595