fix DCE + va_arg problem

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22264 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Instructions.h

@@ -681,6 +681,7 @@ public:
   }
 
   virtual VAArgInst *clone() const;
+  bool mayWriteToMemory() const { return true; }
 
   // Methods for support type inquiry through isa, cast, and dyn_cast:
   static inline bool classof(const VAArgInst *) { return true; }