SIMPLE: A language-independent tree IR

[Joe Buck]

Diego writes:

> Yes.  My initial plan is to model the different tree -> SIMPLE
> converters after the existing tree -> RTL ones.  All the
> language semantics are encoded in the current expanders, so it
> should be a matter of making the expanders output SIMPLE trees
> instead of RTL.
> 
> I'm not actually sure how true the above is.  We may have some
> semantics encoded in other places.  Once the semantics have been
> explicitly expanded into SIMPLE, then we can have a single
> SIMPLE->RTL expander.

It seems that nodes could be added to represent sequence points,
but alias sets would have to be represented in some other way.