WiP check the IR after the optimizer, does not work.

This actually does not work at all, because the optimizer does
produce invalid trees, as I expected/feared. The most common
error that arises:

Cannot select someField of non-class SomeTrait

which happens typically because looking up the accessor of
someField in the instantiated subclasses of SomeTrait gives
only one target, which contains the Select of the field. That
target is then inlined, and then the thing does not typecheck
anymore.