We don't sink open_existential_* instructions, because there may be some instructions depending on them, e.g. ametatype_inst, etc. But this kind of dependency cannot be expressed in SIL as a use yet. As a result, sinking open_existential may break the invariant that these instructions should dominate their uses.
(libraries now)
It has been generally agreed that we need to do this reorg, and now
seems like the perfect time. Some major pass reorganization is in the
works.
This does not have to be the final word on the matter. The consensus
among those working on the code is that it's much better than what we
had and a better starting point for future bike shedding.
Note that the previous organization was designed to allow separate
analysis and optimization libraries. It turns out this is an
artificial distinction and not an important goal.
