Remove fake borrows of refs that are converted into non-refs in MakeByMoveBody

[compiler-errors]

Remove fake borrows of closure captures if that capture has been replaced with a by-move version of that capture.

For example, given an async closure that looks like:

let f: Foo;
let c = async move || {
    match f { ... }
};

... in this pair of coroutine-closure + coroutine, we capture Foo in the parent and &Foo in the child. We will emit two fake borrows like:

_2 = &fake shallow (*(_1.0: &Foo));
_3 = &fake shallow (_1.0: &Foo);

However, since the by-move-body transform is responsible for replacing _1.0: &Foo with _1.0: Foo (since the AsyncFnOnce coroutine will own Foo by value), that makes the second fake borrow obsolete since we never have an upvar of type &Foo, and we should replace it with a nop.

As a side-note, we don't actually even care about fake borrows here at all since they're fully a MIR borrowck artifact, and we don't need to borrowck by-move MIR bodies. But it's best to preserve as much as we can between these two bodies :)

Fixes #138501

r? oli-obk

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[oli-obk]

@bors r+ rollup

[bors]

📌 Commit e54bde6 has been approved by oli-obk

It is now in the queue for this repository.