This is far from obvious -- especially once SIGPIPE comes into play, it's quite possible that printf will terminate the program and prevent the undefined behavior from occurring. Which means the compiler is not allowed to optimize it out.
The only issue is that writing to a stream is visible behavior. I believe that it would still be okay to eliminate visible behavior if the program asserts that it's unreachable. The only reason you might not be able to coax the elimination out of compilers is that they are being careful around visible behavior. (Or, more weakly, around external function calls).