Take first steps towards simplifying the constraint graph by performing edge contraction.

lib/Sema/ConstraintGraph.h

@@ -287,6 +287,10 @@ public:
   /// Verify the invariants of the graph.
   void verify();
 
+  /// Optimize the constraint graph by eliminating simple transitive
+  /// connections between nodes.
+  void optimize();
+
 private:
   /// Remove the node corresponding to the given type variable.
   ///
@@ -304,6 +308,15 @@ private:
   /// caution.
   void unbindTypeVariable(TypeVariableType *typeVar, Type fixedType);
 
+
+  /// Perform edge contraction on the constraint graph, merging equivalence
+  /// classes until a fixed point is reached.
+  bool contractEdges();
+
+  /// To support edge contraction, remove a constraint from both the constraint
+  /// graph and its enclosing constraint system.
+  void removeEdge(Constraint *constraint);
+
   /// The constraint system.
   ConstraintSystem &CS;