Every phase ought to have textual output for debugging

As far as I can tell, currently `--ir-write-all` prints:
- core (optimized or not depends on an optimization toggle)
- target
- machine (if relevant for the given target)

I'd like a much more fine-grained solution. For many bugs, it would be much easier if we could print the current state before/after every phase like:
- before/after Polymorphism Boxing
- before/after individual optimization/normalization passes
- before/after CPS on JS backend
- before all core optimizations and after all core optimizations
- etc

My goal is to be able to call some `--ir-write-all-yes-really` and get 382 files (one for each nanopass) I can binary search through in order to determine what went wrong in which phase, without needing to modify the compiler, sparkling debug prints everywhere.
An example is #934 — I think I know where the issue is, but I really don't want to ad hoc trace the whole JS backend code...