Auto merge of #124153 - scottmcm:more-placevalue, r=saethlin

Refactoring after the `PlaceValue` addition

I added [`PlaceValue`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/place/struct.PlaceValue.html) in #123775, but kept that one line-by-line simple because it touched so many places.

This goes through to add more helpers & docs, and change some `PlaceRef` to `PlaceValue` where the type didn't need to be included.

No behaviour changes -- the codegen is exactly the same.

Author: bors