Investigate reworking CBR and RBR to match Dream

[frabert]

From #288

The new condition-based refinement pass is fine IMO. Better and simpler than the previous version, but might miss some structurization opportunities compared to the version in No More Goto's or the Dream decompiler, which does clustering across the whole compound statement, but also is aware of reachability and probably side-effects too in some way.

The reach-based refine seems conceptually similar to me, but might have correctness issues that I mentioned in the comments in the code.