[tree-ssa] Garbage collector x bb annotations

[S. Bosscher]

> > The real solution would be to
> > finally convert basic blocks to ggc.  Volunteers?
> 
> I may try (I need it anyway for tree-ssa-cfg branch).  The whole issue
> is however quite ugly as this combines basically all allocation
> mechanisms that are available in gcc (basic blocks themselves are pool
> allocated).

We already mix everything now: bitmaps that are xmalloc'ed, annotations are
ggc'ed, and bbs and edges are pool allocated.  Talk about fragile...
BBs and edges and everything attacked to it should also be garbage
collected.  If we ever go on to save functions as graphs then that would be
more convenient anyway.

Gr.
Steven