[tree-ssa] Unlink_block x gc

[Zdenek Dvorak]

Hello,

> > prev_bb and next_bb are marked as chain_next and chain_prev; thus they
> > must always form a double-linked list, and therefore we must reset them
> > in the unlink_block.
> > 
> > Commited as obvious.
> 
> Why the unliked basic block remainied reachable to the garbage
> collector?  Is there some sane reason for referencing dead basic block?

if you do ggc_push_context and then call ggc_collect, this happens (I
noticed this during testing my new induction variable optimizer, in that
I have to use this construction since just now I produce great amounts
of junk).

Zdenek