2nd combinator parser example

[Matthew Fluet]

> > > I have isolated the bug to one of our SSA optimization passes.  For
> > > now, you can avoid the bug by compiling with "-drop-pass knownCase",
> > > which turns off the buggy pass.
> > 
> > Matthew, are you interested to take a look?
> 
> Yeah, I'll take a look at it this morning.

I identified the cause of the bug and have a good idea what the right fix
should be.  It'll take a little hacking to get it done and tested.