Perform constant optimizations for integer division and modulus on the C-- level.

This way we can avoid having to duplicate the same functionality for
every backend, and we may also benefit from other optimizations performed
during C-- generation.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14303 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02