A 62-year-old man with a history of ischemic cardiomyopathy (left ventricular ejection fraction of 30%), hypertension, and hyperlipidemia presents for routine follow-up. He reports mild exertional dyspnea on climbing one flight of stairs but denies orthopnea, paroxysmal nocturnal dyspnea, or lower extremity edema. His current medications include lisinopril, furosemide, and atorvastatin. Physical examination shows a blood pressure of 118/70 mm Hg, pulse of 68 beats/min, and clear lung fields. Laboratory tests reveal stable renal function and potassium of 4.2 mEq/L. Which one of the following medications, when added to his regimen, has been shown in large randomized trials to reduce mortality in patients with stable heart failure with reduced ejection fraction?