Fix interproc constant folding with side-effects

[t1mlange]

There's no guarantee that the constant folding actually folds in the inserted assign.

[StevenArzt]

In most cases, the resulting Jimple code should still be the same because we run a DeadCodeEliminator over the simplified Jimple bodies in the end if I am not mistaken. So just not removing the assignment here should be the safest option.