Cyclic dependencies in instruction selector

[philzook58]

An issue addressed in the Blindell paper is that sometimes patterns can arrange themselves in such a way that there is a cyclic dependency that cannot actually be scheduled. These cycles need to be found and broken. It is my anticipation that Vibes will fail to solve instruction scheduling if it finds such a dependency. I do not expect to run into this issue until we start using more complex patterns, so this is put on hold for now