My advisor once said "Optimization means: sometimes, it is not getting worse", and that's excactly what happens if you perform any optimization on IR level. You don't have any guarantee that your result is optimal (regarding whatever). That's exactly the reason why Massalin introduced the term superoptimization, because on machine level you can provide such guarantees for the considered instruction sequence.

Of course there a cases where preventing an IR optimization can trigger a more powerful one and the resulting code gets better. However, in general I would say it's the other way around: Performing the IR superoptimization leads to better code quality.