We a 12G Tank with 12G of actual water in it will weigh 100lbs in water alone. the tank itself and accessories, you'll be looking at another 40lbs. You'll substitute some of the 12G with subtrate, which will weigh about 20lbs for that size edge, mind you that displaces some water. I would expect that whatever decorations you place in the tank would offset the water weight similarly on average, unless you go ALL rock. So I think 160lbs is a good potential prediction of total weight.
The Ikea shelf says 33lbs/shelf on their site. If we use the middle 2 sections, which yields 4 compartments and the top shelf, I would assume that you could do 4x33lbs=132lbs, plus the top shelf which should do twice the weight of 33lbs in that section (66+132=198lbs).
I would say based on that info that you "should" be safe. I would minimize load on other areas of the unit and avoid using the middle compartments for anything other than light supplies like fish food, ferts etc.
Hope this helps.