Marking isRoot function as const

hpvm/include/SupportVISC/DFGraph.h

@@ -313,7 +313,7 @@ public:
       DFInternalNode* _Parent, unsigned _NumOfDim, std::vector<Value*>
       _DimLimits, DFNodeKind _K);
 
-  bool isRoot() {
+  bool isRoot() const {
     // It is a root node is it was created from a launch intrinsic
     return II->getCalledFunction()->getName().equals("llvm.visc.launch");
   }