Slightly generalize AddCheckConstantFunction.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 2ebf7f94fd0c20c89355577e22ab1135ea02b7f6..a2aea52b8b152246ca2bf1d3f2b4fc8d3aa4d1d1 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -1584,7 +1584,12 @@ class Call: public Expression {
   virtual bool IsMonomorphic() { return is_monomorphic_; }
   CheckType check_type() const { return check_type_; }
   Handle<JSFunction> target() { return target_; }
+
+  // A cache for the holder, set as a side effect of computing the target of the
+  // call. Note that it contains the null handle when the receiver is the same
+  // as the holder!
   Handle<JSObject> holder() { return holder_; }
+
   Handle<JSGlobalPropertyCell> cell() { return cell_; }
 
   bool ComputeTarget(Handle<Map> type, Handle<String> name);