[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

llvm-svn: 219207
diff --git a/polly/lib/Support/ScopHelper.cpp b/polly/lib/Support/ScopHelper.cpp
index 1aec052..87ee1ab 100644
--- a/polly/lib/Support/ScopHelper.cpp
+++ b/polly/lib/Support/ScopHelper.cpp
@@ -123,8 +123,8 @@
     // inserted __after__ EntryBB causing the following situation:
     //
     // EnteringBB
-    //     |
-    //    / \
+    //    _|_
+    //    | |
     //    |  \-> some_other_BB_not_in_R
     //    V
     // EntryBB