Add missing doxygen comment

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@200410 91177308-0d34-0410-b5e6-96231b3b80d8
ScopDetect: Transitively remove all children after region expansion

In rare cases, a region R which is itself not valid has an indirect child region
that is valid. When R becomes part of a valid region by expansion of another
region, then all children of R have to be erased from the set of valid regions.
This patch ensures that indirect children are erased in addition to direct
children.

Contributed-by: Armin Groesslinger <armin.groesslinger@uni-passau.de>

Tobias: I added a reduced test case and adjusted the logic of the patch to
        only recurse until the first child is found.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@200411 91177308-0d34-0410-b5e6-96231b3b80d8