Before I commit this to master, I was thinking that this patch looks safe enough for the maint branch. If there are no objections, I can commit this to maint and merge to master.

The patch adds a new function org-invisible-p whose definition is the same as outline-invisible-p from emacs 25.2 and refactors all instances of outline-invisible-p with org-invisible-p. So the change will be transparent for people using emacs stable releases.

But as a plus, people using emacs master + org maint will get this bug fix too.