Combination of the Dipeptidyl Peptidase IV Inhibitor LAF237 [(S)-1-[(3-Hydroxy-1-adamantyl)ammo]acetyl-2-cyanopyrrolidine] with the Angiotensin II Type 1 Receptor Antagonist Valsartan [N-(1-Oxopentyl)-N-[[2′-(1 H-tetrazol-5-yl)-[1,1′-biphenyl]-4-yl]methyl]-l-valine] Enhances Pancreatic Islet Morphology and Function in a Mouse Model of Type 2 Diabetes