New back-end optimization pass: common subexpression elimination (CSE).
(Reuses results of previous computations instead of recomputing them.)
(Cherry-picked from branch backend-optim.)
Tested on amd64/linux and i386/linux.
Other back-ends compile (after assorted updates) but are untested.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14688 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02