limit checks

[Stephen Weeks]

> They can be hoisted to coincide with the stack check at function
> entry.  So, statically, we didn't gain much (3 lcs in the function
> down to 1 lc).

I didn't put in the code yet to combine the heap checks with the stack
check (it's on the todo, but nowhere near the top, so do it if you
want).  The stack check is always inserted separately as the first
thing in the function.  So what's probably going on is 3 -> 2, not 3
-> 1.