limit check insertion

[Stephen Weeks]

> With regards to exposing address arithmetic (in array references and
> the like) this strikes me as a big win.  Some day we want to have
> things like replacing array subscripting with induction variables
> that are pointers, and even barring that it is probably not hard to
> expose the common sub-expression which is multiplying by array
> element size in something like an array copy.

Yeah, it will be interesting to try to get around the problems caused
with interactions between the gc and having pointers into an array --
and even expressing that in the type system.