Auto merge of #61872 - matthewjasper:refactor-mir-drop-gen, r=nikomatsakis

Clean up MIR drop generation

* Don't assign twice to the destination of a `while` loop containing a `break` expression
* Use `as_temp` to evaluate statement expression
* Avoid consecutive `StorageLive`s for the condition of a `while` loop
* Unify `return`, `break` and `continue` handling, and move it to `scopes.rs`
* Make some of the `scopes.rs` internals private
* Don't use `Place`s that are always `Local`s in MIR drop generation

Closes #42371
Closes #61579
Closes #61731
Closes #61834
Closes #61910
Closes #62115