Fix off-by-one in Interpreter::getFirstVarArg(), which was punishing

any attempts by LLI to use varargs (possibly left over from the introduction
of IntrinsicLowering??)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11370 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ExecutionEngine/Interpreter/Interpreter.h

@@ -159,7 +159,7 @@ public:
   }
 
   GenericValue *getFirstVarArg () {
-    return &(ECStack[ECStack.size () - 2].VarArgs[0]);
+    return &(ECStack.back ().VarArgs[0]);
   }
 
   //FIXME: private: