Inconsistent behavior with dead entities

The current behavior of dead entities is that they cannot be targeted by selectors. However, predicates can still detect them as vehicles/passengers, which means there is currently no reliable way to test for certain entities that have just been killed. I propose one of several solutions:

1. Add a sub-command "execute on victim" that, when ran from a "player_killed_entity" advancement, can change context to the entity who was just killed.
2. Change targeting of dead entities to be allowed, preferably through the use of a predicate flag.
3. Fix the inconsistent behavior of predicates checking for existing vehicles/passengers.