Just scalarize trunc stores - GenWidenVectorTruncStores does the same thing but is flawed (PR42046) and unused.
This is by inspection - as was mentioned on the ticket, this code isn't active at all in the tests and I've not been able to get it to fire
In theory, you just need the right kind of node. That said, DAGCombiner specifically avoids bulding such a node, so not sure it ever happens in practice. I think I'm okay with this as-is.