Auto merge of #51773 - oli-obk:cleanup_impl_trait, r=nikomatsakis

Don't inspect the generated existential type items

r? @nikomatsakis

My debugging led me to the `hir::ItemExistential(..)` checks, which are entirely unnecessary because we never use the items directly. The issue was that items were iterated over in a random order (due to hashmaps), so if you checked the `ItemExistential` before the function that has the actual return `impl Trait`, you'd run into those ICEs you encountered.