I think it is a bad idea to do so.
We need a clear syntax to know if the SIMD would be applied.
I think a function would be overkill though. It's just multiple adding or subtracting happening at the same time, so it would be like having a function for adding 2 floats and a function for adding 2 ints and so on. And most compilers already optimize SIMD into a lot of situations, so the cases where SIMD is used don't have to be obvious.