Detection: Allow to filter the regions that can be detected

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@200224 91177308-0d34-0410-b5e6-96231b3b80d8
TODO: Add topic 'teach bugpoint to extract regions'

This does not only seem helpful for Polly, but it should help in general to
further reduce bugs.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@200225 91177308-0d34-0410-b5e6-96231b3b80d8