Modify SimplifyArmIdentity so it can trigger on mir-opt-level=1

I also added test cases to make sure the optimization can fire on all of
these cases:

```rust
fn case_1(o: Option<u8>) -> Option<u8> {
  match o {
    Some(u) => Some(u),
    None => None,
  }
}

fn case2(r: Result<u8, i32>) -> Result<u8, i32> {
  match r {
    Ok(u) => Ok(u),
    Err(i) => Err(i),
  }
}

fn case3(r: Result<u8, i32>) -> Result<u8, i32> {
  let u = r?;
  Ok(u)
}

```

Without MIR inlining, this still does not completely optimize away the
`?` operator because the `Try::into_result()`, `From::from()` and
`Try::from_error()` calls still exist. This does move us a bit closer to
that goal though because:

- We can now run the pass on mir-opt-level=1

- We no longer depend on the copy propagation pass running which is
  unlikely to stabilize anytime soon.

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