We have special code to handle field flow for single-field structs, but that special case was too specific. Some `Store`s to single-field structs have no `Chi` instruction, which is the case that we handled already. However, it is possible for the `Store` to have a `Chi` instruction (e.g. for `{AllAliased}`), but still have a use of the result of the `Store` directly. We now add a `PostUpdateNode` for the result of the `Store` itself in those cases, just like we already did if the `Store` had no `Chi`.
This consistency check seems to have value for AST data flow, but I've
disabled it on the IR for now.
This commit also includes two unrelated changes that seem to fix a
semantic merge conflict.
