Fix tree-ssa-dce Ada LTO bootstrap ice

[Jan Hubicka]

> Jan,
> 
> > this patch fixes tree-ssa-dce ICE seen during Ada bootstrap.  It is updated
> > version of https://gcc.gnu.org/ml/gcc-patches/2015-05/msg02876.html for
> > current mainline
> > 
> > Bootstrapped/regtested x86_64-linux, OK?
> > 
> > 	PR middle-end/65337
> > 	* tree-ssa-dce.c (bb_postorder): New static var.
> > 	(forward_edge_to_pdom): Remove.
> > 	(remove_dead_stmt): Instead of redirecting edges only keep an edge
> > 	on a path to nearest live BB.
> > 	(eliminate_unnecessary_stmts): Free bb_postorder.
> > 	* cfganal.c (dfs_find_deadend): Add START_POINTES.
> > 	* cfganal.h (inverted_post_order_compute): Update prototype.
> 
> do you plan to apply it in the near future?  It's probably too late for GCC 5 
> at this point, but it would be nice to have it for GCC 6.

Eric,
I did not notice the ack from Richard.  I will re-test and commit it ASAP
(probalby tomorrow).

Honza