Introduce register dump helper
MachineVerifier: Report register for bad liveranges
Add a print and verify pass after the RegisterCoalescer
RegisterCoalescer: Improve debug messages

- Show "Considering..." message after flipping so you actually see the final
  destination vreg as destination.
- Add a message on final join, so you can grep for "Success" messages to obtain
  a list of which register got merged with which.