ssa restore

[Matthew Fluet]

> > Something else to keep in mind for the exceptions paper:
> > without needing to worry about HandlerPush/Pops, I would just have
> > rewriteTransfer map route over all the labels.  Now, with the old version
> > of route, this would have given each nontail call at a handler with phi
> > arguments a different route block.  Nothing in the simplification pipeline
> > would eliminate these alpha-equivalent blocks, so implementHandlers would
> > probably do something costly, particularly in terms of space.
> 
> Maybe this should go on the common-block todo list?

Maybe; although looking for alpha-equivalent blocks is non-trivial
(although, I've always wanted to do it, because lots of splits between
Line and Buffered IO lead to alpha-equivalent blocks).