self compile problems with SSA simplifier

[Stephen Weeks]

> I forgot to mention the other problem, which was that commonSubexp
> raised Overflow when self-compiling.  I'm not sure whether it's that
> pass or the shrinker.

I figured this one out, but haven't yet fixed it.  The problem wasn't
in the SSA simplifier at all -- it was in the property list numLinks
counter!  Anyways, I checked in some better error reporting in
property lists so this is at least easy to catch in the future.

The real problem is in some property list leaks in the SSA
simplifier (if you compile -v3 you will see them).  I started working
on those, but haven't yet finished.  Hopefully tomorrow or Monday.