[Refactor] Change the comment style to silence -Wcomment

  -Wcomment complained about a "multi-line comment" caused by the
  ascii art used in ScopHelper to describe the CFG.

Differential Revision: http://reviews.llvm.org/D5618


git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@219207 91177308-0d34-0410-b5e6-96231b3b80d8