Simple, but very effective optimization.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15513 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Undo previous commit.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15514 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02