Make use of boxity analysis in SpecConstr.
It seems we put a lot of work into figuring out already if unboxing an argument is safe to do or not.
We should make use of that for SpecConstr which currently will always take apart boxed arguments if it can.
The main difference for SpecConstr is that we also want to apply this logic to lazy arguments. I'm not sure how hard that would be to achieve.
But even just not taking apart strict arguments if boxity indicates we shouldn't would be an improvement.