Storing a trivial enum case in a non-trivial enum must be treated like a non-trivial init or assign, e.g.
```
%1 = enum $Optional<String>, #Optional.none!enumelt
store %1 to [trivial] %0 // <- cannot delete this store!
store %2 to [assign] %0
```
To make it available in other optimizations as well.
Also, a few problems:
* Use destructre instructions when in OSSA
* Don't split the store if it's nominal type has unreferenceable stoarge
* rename it to `trySplit` because it's not guaranteed to work
Also, add the counterpart for load instructions: `LoadInst.trySplit()`
`ownership` is a bad name in `LoadInst`, because it hides `Value.ownership`.
Therefore rename it to `loadOwnership`.
Do the same for ownership in StoreInst to be consistent.
The old C++ pass didn't catch a few cases.
Also:
* The new pass is significantly simpler: it doesn't perform dataflow for _all_ memory locations at once using bitfields, but handles each store separately. (In both implementations there is a complexity limit in place to avoid quadratic complexity)
* The new pass works with OSSA
