be explicit when constructing integers that are block headers

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14464 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
improved abstractions for register naming in the native code backend

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14465 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02