A 65-year-old man suffered an anterior myocardial infarction (MI) 4 weeks ago. An echocardiogram performed prior to discharge showed impaired left ventricular systolic function with an ejection fraction (LVEF) of 35%. He is currently taking aspirin, clopidogrel, atorvastatin, and bisoprolol. Which additional class of medication is most strongly indicated to improve mortality in this patient?