stupid limit check in fib.sml

[Stephen Weeks]

> Anyways, looking at limit-checks should probably go on the "SSA todo"
> list; I think that insert limit-checks will sometimes hoist limit checks
> into non-allocating loops, which is bad, for the comparisions above.  

Yep.  It's on the todo.

However, that's not the problem here.  The problem is that both a stack limit
check and a heap limit check is always inserted at the beginning of a function.
Period.  In the machine IL, LimitCheck.Stack means do both a heap and a stack
limit check.  That should be fixed.