Hi Max,
Yes, a pants suit would be just fine, and a simple dress or pants for the casual nights. We went on Infinity in Alaska last summer and I did wear pants for the casual nights and a nice dress for the semi-formal, the formal nights are what I've seen on all the other cruises, just that formal.
Between the regular suitcases, we also take a garment bag for some of the dressier clothes and always do pack a different outfit for each night, you may be able to mix/match tops/pants, that can save some packing space.
Enjoy your Celebrity cruise in Alaska, you'll love it!