Remove builtin_unreachable in VRP

Remove builtin_unreachable in VRP

Removal of __builtin_unreachable calls were handled in an inconsistent
way.  This removes then in the VRP pass, and sets the global range
appropriately.

	* tree-vrp.cc (class remove_unreachable): New.
	(remove_unreachable::maybe_register_block): New.
	(remove_unreachable::remove_and_update_globals): New.
	(rvrp_folder::rvrp_folder): Initialize m_unreachable.
	(rvrp_folder::post_fold_bb): Maybe register unreachable block.
	(rvrp_folder::m_unreachable): New member.
	(execute_ranger_vrp): Add final_pass flag, remove unreachables.