ScopDetection: Make enum function-local

The 'Color' enum is only used for irreducible control flow detection. Johannes
already moved this enum in r270054 from ScopDetection.h to ScopDetection.cpp to
limit its scope to a single cpp file. We now move it into the only function
where this enum is needed to make clear that it is only needed locally in this
single function.

Thanks to Johannes for pointing out this cleanup opportunity.

llvm-svn: 272462
diff --git a/polly/lib/Analysis/ScopDetection.cpp b/polly/lib/Analysis/ScopDetection.cpp
index 5cc3336..c4bb515 100644
--- a/polly/lib/Analysis/ScopDetection.cpp
+++ b/polly/lib/Analysis/ScopDetection.cpp
@@ -1380,14 +1380,14 @@
   }
 }
 
-/// @brief Enum for coloring BBs in Region.
-///
-/// WHITE - Unvisited BB in DFS walk.
-/// GREY - BBs which are currently on the DFS stack for processing.
-/// BLACK - Visited and completely processed BB.
-enum Color { WHITE, GREY, BLACK };
-
 bool ScopDetection::isReducibleRegion(Region &R, DebugLoc &DbgLoc) const {
+  /// @brief Enum for coloring BBs in Region.
+  ///
+  /// WHITE - Unvisited BB in DFS walk.
+  /// GREY - BBs which are currently on the DFS stack for processing.
+  /// BLACK - Visited and completely processed BB.
+  enum Color { WHITE, GREY, BLACK };
+
   BasicBlock *REntry = R.getEntry();
   BasicBlock *RExit = R.getExit();
   // Map to match the color of a BasicBlock during the DFS walk.