isSafeToSpeculateStore() looks for a preceding store to the same location to make sure that introducing a new store of the same value is safe. It currently bails on intervening mayHaveSideEffect() instructions. However, I believe just checking mayWriteToMemory() is sufficient there -- we just need to make sure that we know which value was stored, we don't care if we can unwind in the meantime.
While looking into this, I started having some doubts about the correctness of the transform with regard to thread safety. While we don't try to hoist non-simple stores, I believe we also need to make sure that the preceding store is simple as well. Otherwise we could introduce a spurious non-atomic write after an atomic write -- I believe under our memory model this would result in a subsequent undef atomic read, even if the second write stores the same value as the first.