In language 6 mode keypath instructions are created as existentials and the optimizer needs to look through the `open_existential_ref` instructions to recognize a keypath.
rdar://150173106
Handle open_existential_ref instructions which cast keypath instructions before they are passed to a partial_apply.
In Swift language mode 6 keypaths are existentials (e.g. `any WritableKeyPath<Str, Int> & Sendable`) and we need to deal with that in CapturePropagation.
rdar://141370412
This optimizes keypath-closures, like
```
a.map { \.x }
```
It results in a significant performance improvement for such code patterns.
rdar://87968067
