New back-end optimization pass: dead code elimination.
(Removes arithmetic and load instructions whose results are unused.)
(Cherry-picked from branch backend-optim.)


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14686 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
ID comments are so passé.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14687 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02