SSA IL

[Matthew Fluet]

> > Is there a particular reason why the set of SSA functions is a list rather
> > than a vector?
> 
> Yes.  Space safety.  Because when translating from one program to another, one
> function at a time, Vector.map and Vector.fold keep alive the whole vector,
> while the corresponding List functions only keep alive the tail.  That space
> leak can hurt on big programs.

Ahh...that makes sense.

I'm just about done with removeUnused.  A few semanitcs questions:

1. in the CPS version, we don't selectively include Dec.Bind's depending
on the VarInfo.t of the var; seems that the only thing affected by the
VarInfo.t's are globals -- maybe that's correct; unused Dec.Bind's are
dead code, and eliminated by shrinks; but globals aren't eliminated by
shrink.

2. when should I visit a handler?  At a setHandler or at a use of a
handler at a non-tail call?  I think the latter; the use of a handler at a
non-tail call should be dominated by a setHandler of the handler; but we
can always setHandlers that aren't ever used; something (shrinker?) should
delete setHandler's for labels that don't have any corresponding non-tail
calls.