Preserve narrowing in unreachable code (#20710)

In the future, it would be nice to have an option to continue to check
unreachable code. See #18707 from A5rocks. This PR lays some semantics
preserving groundwork for that.

This is effectively a subset of #18707 , but I go just a little bit
further and change the type of TypeMap to exclude None

Btw A5rocks if you are willing to rebase your PR, that would be great. I
think `--check-unreachable` will be a very useful feature.

Co-authored-by: A5rocks

Author: hauntsaninja