Speedup update-ssa some more

The following avoids copying an sbitmap and one traversal by avoiding
to re-allocate old_ssa_names when not necessary.  In addition this
actually checks what the comment before PHI insert iterating promises,
that the old_ssa_names set does not grow.

	* tree-into-ssa.cc (iterating_old_ssa_names): New.
	(add_new_name_mapping): Grow {new,old}_ssa_names separately
	and only when actually needed.  Assert we are not growing
	the old_ssa_names set when iterating over it.
	(update_ssa): Remove old_ssa_names copying and empty_p
	query, note we are iterating over it and expect no set changes.