RSSA backend

[Matthew Fluet]

> > > 4. make the array initialization loops explicit in MACHINE; I guess this
> > >     would be an insertArrayInitializations after insertSignalChecks
> > > 
> > > I think we eventually want 4, which solves all the problems at once.
> > 
> > Agreed.  I will do it right now.  It should be easy.
> 
> I checked in the array initialization stuff.  

I like making the array initialization explicit in RSSA, but,
unfortunately, it doesn't solve the whole problem.  I forgot that the
control flow to update the frontier based on whether or not numElts is
zero requires branches (and hence, new labels).  So the problem is
still there, although the fact that liveness info doesn't need to be
propagated around a loop means that it should be really quick to fill
those in.  Or, propagate it down from the live list in the entry.