Intended meaning of nubBy use in GHC's compiler/cmm/Debug.hs

[David Feuer]

The meaning of nubBy when applied to functions other than equivalence 
relations changed around the time you created compiler/cmm/Debug.hs. This 
makes it extra tricky to figure out exactly what that nubBy is expected to do. 
Do you think you could explain? Furthermore, it would be helpful to know what 
(if anything) is known about the relationships between the source spans in the 
the argument to the nubBy. This code is proving to be a bit of a hot spot in 
#11095 and the better we understand what's going on the better our chances of 
coming up with good solutions.

Thanks,
David Feuer