More last_stmt removal

More last_stmt removal

This is the last set of changes removing calls to last_stmt in favor of
*gsi_last_bb where this is obviously correct.  As with the last changes
I tried to cleanup the code as far as dependences are concerned.

	* tree-ssa-loop-split.cc (split_at_bb_p): Avoid last_stmt.
	(patch_loop_exit): Likewise.
	(connect_loops): Likewise.
	(split_loop): Likewise.
	(control_dep_semi_invariant_p): Likewise.
	(do_split_loop_on_cond): Likewise.
	(split_loop_on_cond): Likewise.
	* tree-ssa-loop-unswitch.cc (find_unswitching_predicates_for_bb):
	Likewise.
	(simplify_loop_version): Likewise.
	(evaluate_bbs): Likewise.
	(find_loop_guard): Likewise.
	(clean_up_after_unswitching): Likewise.
	* tree-ssa-math-opts.cc (maybe_optimize_guarding_check):
	Likewise.
	(optimize_spaceship): Take a gcond * argument, avoid
	last_stmt.
	(math_opts_dom_walker::after_dom_children): Adjust call to
	optimize_spaceship.
	* tree-vrp.cc (maybe_set_nonzero_bits): Avoid last_stmt.
	* value-pointer-equiv.cc (pointer_equiv_analyzer::visit_edge):
	Likewise.