[Polly][Refactor] Change the comment style to silence -Wcomment

[jdoerfert]

Summary

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

[jdoerfert]

Closed by commit rL219207 (authored by @jdoerfert).

polly/trunk/lib/Support/ScopHelper.cpp

if (EnteringBB->getTerminator()->getNumSuccessors() != 1) {
 
     // Once the edge between EnteringBB and EntryBB is split, two cases arise.
     // The first is simple. The new block is inserted between EnteringBB and
     // EntryBB. In this case no further action is needed. However it might
     // happen (if the splitted edge is not critical) that the new block is
     // inserted __after__ EntryBB causing the following situation:
     //
     // EnteringBB
-    //     |
-    //    / \
+    //    _|_
+    //    | |
     //    |  \-> some_other_BB_not_in_R
     //    V
     // EntryBB
     //    |
     //    V
     // SplitEdgeBB
     //
     // In this case we need to swap the role of EntryBB and SplitEdgeBB.