Handle use of a better calling convention for combined forward/reverse

[wsmoses]

There are instances in which a we need a shadow pointer from a function we can create a combined forward/reverse of (See #27). However, right now if that is the case we are conservative and fall back to individual forward/reverse and we should make sure that uses of the inverted pointer are moved to the right location when using the combined variant (and re-enable it in these cases).