Based on discussion in #37827, we could further improve typing of this-property assignments if we used autoType whenever we can't tell what type they should have.

This has three open questions:

1. Performance.
2. Is there benefit in real code? This only helps if (1) we aren't getting the right type some of time where (2) the auto type would get the right type. This is likely to be when a property is used repeatedly in a long method.
3. What changes are actually needed. Just returning autoType doesn't work, because control flow narrowing doesn't seem to track this-property assignments as references.