This issue came up in Trac #11480, and is documented in
Note [When superclasses help] in TcRnTypes.

We were getting a spurious warning
  T11480.hs:1:1: warning:
     solveWanteds: too many iterations (limit = 4)

The fix is easy.  A bit of refactoring along the way.

The original bug report in Trac #11480 appears to work
fine in HEAD and the 8.0 branch but I added a regression
test in this commit as well.