Tiling on sheetrock around a shower is a disaster waiting to happen. Especially if it doesn't have a vapor barrier behind it. Also, your tub deck may not have a tiling flange, and water on the rim will just get back into the walls. You really want something that won't degrade from the water spray such as a cbu instead of sheetrock. If the tub does have a tiling flange, you could make it work. The tub company (and there are generic ones) may have an add-on tiling flange, if you don't.
So, it could be a real mess, depending on what you're starting with. You have a couple choices on the shower...add a diverter valve or change the tub spout to one with a diverter in it.