SIMPLE: A language-independent tree IR

[Joe Buck]

> - Every front-end rewrites its tree representation into SIMPLE.
>   Sebastian has already implemented some of the simplification
>   steps for the C front-end.

How would structs be handled under this proposal?  One option would
be to explicitly generate the address arithmetic, but that has the
effect of prematurely committing the struct to memory (one of the
biggest reasons for g++'s inferiority compared to KAI).