limit check insertion

[Matthew Fluet]

> > > I would still like to hear opinions on the short term fix of
> > > restricting limit check coalescing to basic blocks.
> > 
> > Meaning, let alloc(l) = sum of allocations in the block and setting
> > limitCheck(l) = alloc(l)?
> 
> Yes, modulo threads and variable size allocations.
> 
> > Sounds cheap and quick (to write); I guess we need to see how it plays out
> > in practice to determine how quickly we need to move to something else.
> > I've got no objections to it.
> 
> I'll start on it tomorrow then.

Sounds good.  I'm off for the night.  I might get to fixing up the backend
tonight.