[ScopInfo] Introduce tryGetValueStored

Summary:
This makes code more readable and allows to reuse this functionality in
the future at other places.

Suggested-by Michael Kruse in post-commit review of r307660.

Reviewers: Meinersbur, bollu, gareevroman, efriedma, huihuiz, sebpop, simbuerg

Reviewed By: Meinersbur

Subscribers: pollydev, llvm-commits

Tags: #polly

Differential Revision: https://reviews.llvm.org/D35585

llvm-svn: 308435