[Reduce] Rewrite function body delta pass again

It is not enough to replace all uses of users of the function with undef,
the users, we only drop instruction users, so they may stick around.

Let's try different approach - first drop bodies for all the functions
we will drop, which should take care of blockaddress issue the previous
rewrite was dealing with; then, after dropping *all* such bodies,
replace remaining uses with undef (thus all the uses are either
outside of functions, or are in kept functions)
and then finally drop functions.

This seems to work, and passes the *existing* test coverage,
but it is possible that a new issue will be discovered later :)

A new (previously crashing) test added.