Fix a bug in mk_superclasses_of
Fix a bug in mk_superclasses_of

This bug meant that we were less eager about expanding
tuple superclasses than we should have been; i.e. we stopped
too soon.  That's not fatal, beause we expand more superclasses
later, but it's less efficient.

shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,olsner/ghc,mettekou/ghc,sdiehl/ghc,olsner/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,mettekou/ghc,shlevy/ghc,shlevy/ghc,mettekou/ghc,ezyang/ghc,olsner/ghc,sdiehl/ghc,olsner/ghc,mettekou/ghc,mettekou/ghc,olsner/ghc,ezyang/ghc,sdiehl/ghc,olsner/ghc,mettekou/ghc,mettekou/ghc,shlevy/ghc,olsner/ghc,ezyang/ghc,shlevy/ghc