Note [Empty case alternatives]

* For a key (Case e b ty (alt:alts))  we don't need to look the return type
  'ty', because every alternative has that type.

* For a key (Case e b ty []) we MUST look at the return type 'ty', because
  otherwise (Case (error () "urk") _ Int  []) would compare equal to
            (Case (error () "urk") _ Bool [])
  which is utterly wrong (#6097)

We could compare the return type regardless, but the wildly common case
is that it's unnecessary, so we have two fields (cm_case and cm_ecase)
for the two possibilities.  Only cm_ecase looks at the type.

See also Note [Empty case alternatives] in GHC.Core.