Avoid unnecessary double visits on equals containing conditional access in nullable

[RikkiGibson]

I seems that when this condition is false, we'll have done the first visit (based on when-not-null state) for nothing.
I suspect we can re-organize the code to do the second visit (based on worst-case state) first, then decide if we need to do the first visit (based on when-not-null state). #Closed

Originally posted by @jcouv in #53198 (comment)

---

It seems like we might be able to refactor the implementation to do the "worst case" visit for diagnostics/public API first, then only if the right side is "known null or not null", would we actually do the second visit to update the "state when not null".

However, I am concerned about brittleness and potentially strange correctness issues when we don't do the "worst case" visit last, so I'd like to punt on this optimization for now.