User type annotations can result in different mir-opt test results in CI vs locally

[davidtwco]

In #73442, a mir-opt test that I blessed locally was failing on CI and I could not reproduce the failure locally - this was due to the DefId in the user type annotations of the test shown below, which appeared to be different locally versus on CI.

// compile-flags: -Z mir-opt-level=1
// Regression test for #72181, this ICE requires `-Z mir-opt-level=1` flags.

use std::mem;

#[derive(Copy, Clone)]
enum Never {}

union Foo {
    a: u64,
    b: Never
}

// EMIT_MIR rustc.foo.mir_map.0.mir
fn foo(xs: [(Never, u32); 1]) -> u32 { xs[0].1 }

// EMIT_MIR rustc.bar.mir_map.0.mir
fn bar([(_, x)]: [(Never, u32); 1]) -> u32 { x }

// EMIT_MIR rustc.main.mir_map.0.mir
fn main() {
    println!("{}", mem::size_of::<Foo>());

    let f = [Foo { a: 42 }, Foo { a: 10 }];
    println!("{:?}", unsafe { f[0].a });
}

You can find my config.toml here and I was running tests with ./x.py test src/test/mir-opt --stage 1 (though I couldn't reproduce with a stage2 build either).

cc @rust-lang/wg-mir-opt