rts/nonmoving: Add missing STM write barrier
When updating a TRec for a TVar already part of a transaction we previously neglected to add the old value to the update remembered set. I suspect this was the cause of #18587 (closed).
When updating a TRec for a TVar already part of a transaction we previously neglected to add the old value to the update remembered set. I suspect this was the cause of #18587 (closed).