Avoid computing RPO for update_ssa

At some point when domwalk got the ability to use RPO for ordering
dominator children we carefully avoided update_ssa eating the cost
of RPO compute.  Unfortunately some later consolidation of CTORs
lost this again so the following makes this explicit via a special
value to the bb_index_to_rpo argument of domwalk, speeding up
update_ssa again.

	* domwalk.h (dom_walker::dom_walker): Update comment to
	reflect reality and new special argument value for
	bb_index_to_rpo.
	* domwalk.cc (dom_walker::dom_walker): Recognize -1
	bb_index_to_rpo.
	* tree-into-ssa.cc
	(rewrite_update_dom_walker::rewrite_update_dom_walker): Tell
	dom_walker to not use RPO.