llvm dev - Jan 2020 - failing branch elimination, how to diagnose

For the backend I am writing, branch elimination is not working with the 
following error message:

Pre-computing triangle chains.
Pre-merging due to unanalyzable fallthrough: %bb.1 ('L.2') -> %bb.2
('return')
     %bb.2 ('return') -> Mid chain!

Obviously I am not constructing the chain correctly.  How do I figure out where 
I went wrong?

thanks,
brian