Contributes to dotnet#79204.

I don't know if this will be considered mergeable, but I wanted to at least try something. For apps that use async a lot (like the Stage2 app we use for Goldilocks), the async infrastructure can cost 10% of the entire executable.

Shuffling a couple things in `GetStateMachineBox` I was able to get 0.23% saving. It's miniscule. In general async is death by a thousand papercuts so I don't see a silver bullet.