SSA form doesn't have a concept of moving into itself (because it doesn't have moves at all), but the reason it might be messy is that other optimization passes

SSA form doesn't have a concept of moving into itself (because it doesn't have moves at all), but the reason it might be messy is that other optimization passes would prefer to stick to their specific jobs and not reimplement every kind of cleaning up after themselves. Once you get past the SSA stages, sometimes you just can't describe the target instructions perfectly enough to emit something clean without cleanup passes.