This adds an additional transform to drop zero-size memcpys, also in the case where the size is only zero after instruction simplification. the motivation is the case from PR54983 where the size is non-trivially zero, and processMemSetMemCpyDependence() keeps trying to reduce the memset size by zero bytes.
I'm a bit unsure on this patch -- it's not really principled. It only works on the premise that if InstSimplify doesn't realize the size is zero, then AA also won't.
The principled approach would be to instead add a isKnownNonZero() guard to the processMemSetMemCpyDependence() transform, but I suspect that would render that optimization mostly useless (at least it breaks all the existing test coverage -- worth noting that the constant size case is also handled by DSE, so I think this transform is primarily about the dynamic size case). Though I can't say I particularly care about this transform either, so I'd be happy to go with that direction instead.