Document why we don't look at storage liveness
Document why we don't look at storage liveness

...when determining what locals are live.

A local cannot be borrowed before it is `storage_live` and
`MaybeBorrowedLocals` already invalidates borrows on `StorageDead`.
Likewise, a local cannot be initialized before it is marked StorageLive
and is marked as uninitialized after `StorageDead`.

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust