[tree-ssa] Garbage collector x bb annotations

[S. Bosscher]

> > me, since basic block annotations are allocated from obstack and
> > invisible to garbage collector.
> > 
> Yeah.  But there's more than that, unfortunately.  All the fields in
> struct bb_ann_d are ggc allocated.  The real solution would be to
> finally convert basic blocks to ggc.  Volunteers?

It has been high on my TODO list for some time, but I wanted to wait until
after 3.4 branches because it will cause changes in a lot of files.  Do you
want it sooner than that?

Gr.
Steven