I would not trim it now since even dead portions help protect a plant from low temperatures and desiccating winds. I would leave it alone until well into the summer when you will know for certain which parts are alive and it will not be damaged by exposing new growth to the weather. Then you can trim off any dead bits. You can also trim it for shape. Use the trimmings in the kitchen.

If you rosemary was suffocated by ice and snow this winter than it is likely to be in very bad shape. I would take a wait and see attitude. In the mean time you may want to source a new plant.