SSA backend

[Stephen Weeks]

> I checked in a fix for the x86-codegen bug.
> 
> I think this works o.k. with my previous remove-unused.fun.

I'm checking it out now.

> However, I'm getting strange errors with this and mllex (and mlyacc).  The
> resulting ssa program type-checks, but register allocation in the backend
> fails. 
...
> I don't claim that remove-unused.new.fun is 100% bug-free, but I also think
> that an ssa program that type checks (i.e., satisfies the SSA condition),
> shouldn't be generating bogus liveness info in the backend.

Yeah, except for Function.checkHandlers, which isn't on in the type checker yet
because I haven't debugged it.  Also, the bug may not be due to
remove-unused.new.fun, it may be due to the fact that there are two passes of
remove at the end of the optimize cycle instead of one.  Anyways, I am
investigating.

> One other thing I added -- to allocate-registers.fun, I made any
> appearance of a Set* statement imply hasHandler.

Makes sense.  It would be nice if removeUnused got rid of the unused Set*
statements.  Although it really doesn't matter since the new approach will be to
infer/insert them after all optimization is done, and that almost certainly
won't introduce bogus ones.