Auto merge of #66282 - Centril:simplify-try, r=oli-obk

[mir-opt] asking `?`s in a more optimized fashion

This PR works towards https://github.com/rust-lang/rust/issues/66234 by providing two optimization passes meant to run in sequence:

- `SimplifyArmIdentity` which transforms something like:
  ```rust
  _LOCAL_TMP = ((_LOCAL_1 as Variant ).FIELD: TY );
  ((_LOCAL_0 as Variant).FIELD: TY) = move _LOCAL_TMP;
  discriminant(_LOCAL_0) = VAR_IDX;
  ```

  into:

  ```rust
  _LOCAL_0 = move _LOCAL_1
  ```

- `SimplifyBranchSame` which transforms `SwitchInt`s to identical basic blocks into a `goto` to the first reachable target.

Together, these are meant to simplify the following into just `res`:
```rust
match res {
    Ok(x) => Ok(x),
    Err(x) => Err(x),
}
```

It should be noted however that the desugaring of `?` includes a function call and so the first pass in this PR relies on inlining to substitute that function call for identity on `x`. Inlining requires `mir-opt-level=2` so this might not have any effect in perf-bot but let's find out.

r? @oli-obk -- This is WIP, but I'd appreciate feedback. :)

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