Add new RegionInfo pass.

The RegionInfo pass detects single entry single exit regions in a function,
where a region is defined as any subgraph that is connected to the remaining
graph at only two spots.
Furthermore an hierarchical region tree is built.
Use it by calling "opt -regions analyze" or "opt -view-regions".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109089 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/GraphWriter.h

@@ -271,6 +271,12 @@ public:
       O << "[" << Attrs << "]";
     O << ";\n";
   }
+
+  /// getOStream - Get the raw output stream into the graph file. Useful to
+  /// write fancy things using addCustomGraphFeatures().
+  raw_ostream &getOStream() {
+    return O;
+  }
 };
 
 template<typename GraphType>