Add description for ClearDFG pass in ClearDFG.cpp

hpvm/lib/Transforms/ClearDFG/ClearDFG.cpp

@@ -6,6 +6,12 @@
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+//
+// This pass HPVM intrinsics from HPVM IR. This pass is the final pass that 
+// runs as a part of clean up after construction of dataflowgraph and LLVM
+// code generation for different targets from the dataflow graph.
+//
+//===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "ClearDFG"
 #include "BuildDFG/BuildDFG.h"