Fix PR#5815

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13789 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Make GADT inference robust to useless equations

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13790 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02