Can US citizen get medication/emergency surgical procedure under tourist visa?
I'm not sure whether you want to travel to Singapore especially for a medical procedure or whether you are worried that you might need to get medical help while on a tourist visa. In either case, it shouldn't matter in your case:
As a US-citizen, you do not require a visa to enter Singapore. You will get a stamp in your passport at arrival with either 30 or 90 days of stay. As long as you don't intend to work in Singapore, this is not bound to a specific purpose, so you can use that to visit a hospital. If you want or need more than the days given to you, you can apply for an extension on medical grounds.
It is fairly common for people to travel to Singapore to seek medical treatment, so most hospitals (here are two examples) will have departments dealing with foreign patients. So if you are unsure, you could contact the hospital in question and they should assist you.