A Multicenter, Randomized, Double-Blind, Double-Dummy, Parallel-Group, 8-Week Comparison of Salmeterol Xinafoate Versus Ipratropium Bromide Versus Salmeterol Xinafoate Plus Ipratropium Bromide Versus Placebo in Subjects With Chronic Obstructive Pulmonary Disease, Trial ID SMS40314 | GSK