Rollup merge of #157035 - amandasystems:region-values-either, r=jackh726

`LivenessValues`: use dedicated enum rather than mutually exclusive `Option`s

I was reading through `LivenessValues` and realised it had a lot of comments explaining that two `Option`s were mutually exclusive, plus some redundant logic to handle this.

This is a drive-by fix to use ~~an `Either`~~ a dedicated enum instead, which makes that property obvious from the code. It also removes the repeated logic.

Author: JonathanBrouwer