Assert that [gs]et_{,r_}cur_block() are only called in phase_building.

ir/ir/ircons.c

@@ -527,6 +527,7 @@ void set_cur_block(ir_node *target)
 
 void set_r_cur_block(ir_graph *irg, ir_node *target)
 {
+	assert(get_irg_phase_state(irg) == phase_building);
 	assert(target == NULL || get_irn_mode(target) == mode_BB);
 	assert(target == NULL || get_irn_irg(target)  == irg);
 	irg->current_block = target;
@@ -534,6 +535,7 @@ void set_r_cur_block(ir_graph *irg, ir_node *target)
 
 ir_node *get_r_cur_block(ir_graph *irg)
 {
+	assert(get_irg_phase_state(irg) == phase_building);
 	return irg->current_block;
 }