"Bad"? You use C because you want speed above all else. If you don't know the language and need hand-holding, don't use C. Compilers are going to optimize C -- that's why it usually runs so fast.
This is bad because the optimizer ignores the imperfect reality about the big crowd of non standard programs. It actually punishes non standard programs (and probably the majority of programs out there are not 100% standard). So it is a bad and patronizing optimization
How many programs are there that depend on SIGNED overflow wrapping around to negative? I can't think of very many - the only places I've seen potential signed overflow that wasn't a bug was when people were checking for overflow.
In those cases, just use unsigned math to do the checks.