I cannot find any evidence that this does not actually work.
The original case here was from Epic's PR series in 2019, but I cannot see why it doesn't work.
It might have been a bug in a very old compiler at some point.
Speculate that we can modify the SSA value in-place. As long as it is
not used after the modify, this is fine.
Also need to make sure we don't attempt to RMW something that is
impossible to modify.
