Fix egregious typo in checkTauTvUpdate.

The old code used an unzonked type in an occurs-check, which
would sometimes lead to an infinite loop.

Please merge to ghc-7.10.

(cherry picked from commit d5cd94d7)

compiler/typecheck/TcUnify.hs

@@ -987,7 +987,7 @@ checkTauTvUpdate dflags tv ty
        ; case sub_k of
            Nothing           -> return Nothing
            Just LT           -> return Nothing
-           _  | is_return_tv -> if tv `elemVarSet` tyVarsOfType ty
+           _  | is_return_tv -> if tv `elemVarSet` tyVarsOfType ty1
                                 then return Nothing
                                 else return (Just ty1)
            _  | defer_me ty1   -- Quick test