[tcb] Incremental SSA updates

[Zdenek Dvorak]

Hello,

> The pass tries to be efficient in only traversing and updating
> just the blocks and statements affected by the name replacement.
> But I still haven't done any profiling.
> 
> I don't know whether this is generic enough to be used in other
> passes that need to update the SSA form.  Besides
> rewrite_ssa_into_ssa, I've seen other functions that may be doing
> similar things.  I'll take a look.

you may also want to check ssaupdate-branch, where I have done something
very similar.

Zdenek