limit checks

[Stephen Weeks]

Matthew, would it cause you any trouble if limit checks became a
MachineOutput.Statement.t and appeared at arbitrary places in the statement list
instead of being forced to occur at the beginning of a block?

Actually, this reminds me, since RCPS is gonna know about object sizes and the
like, it would be nice if we could express limit checks in that IL (an all the
basic blocks associated with them).  It would be even nicer if the "type system" 
could check that the limit checks are sufficient.