ConstProp miscompilation around references

[RalfJung]

#71911 introduced a (nightly-to-nightly) regression in mir-opt-level=3: since that landed, this assertion fails when running the test in Miri.

[RalfJung]

#73223 is also still reproducible with mir-opt-level=2, but @oli-obk said there's a plan to hide it behind a flag that better expresses "this can and will break valid code".

[RalfJung]

In fact even level 2 is broken.

[RalfJung]

... and level 1, for the union issue (so let's treat the other one separately).

[RalfJung]

This doesn't require nightly after all, we have a stable-to-nightly regression when just running this code:

fn main() {
    #[allow(dead_code)]
    union U {
        f1: u32,
        f2: f32,
    }
    let mut u = U { f1: 1 };
    unsafe {
        let b1 = &mut u.f1;
        *b1 = 5;
    }
    assert_eq!(unsafe { u.f1 }, 5);
}

gives

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `5`', src/main.rs:12:5