The answer to that question probably depends on what optimization passes are in the language frontend (as opposed to the language-agnostic backend).
Of course, it's possible that some backend optimizations require proper annotations to be placed by the frontend. Either way though the GCC guys have been pretty good with their C support so I'd be very surprised if the C frontend didn't have the superior optimizer (if they are actually different on their support for the C subset supported by C and C++).
