`optimized_mir` unstable fingerprint when compiling rustc

[Aaron1011]

See #84970 (comment) and #84960 for other instances of this happening

I've managed to reliably reproduce an optimized_mir 'unstable fingerprint' issue when building rustc. Reproduction steps:

1. Clone https://github.com/Aaron1011/rust/ and checkout https://github.com/Aaron1011/rust/tree/optimized-mir-fingerprint
2. Run ./x.py clean followed by ./x.py build --stage 1 library/std
3. Run:

touch compiler/rustc_middle/src/lib.rs

4. Run ./x.py build --stage 1 library/std

You should observe the following error (my config.toml is included in the gist)

https://gist.github.com/Aaron1011/7eec8b2c584f8c9d2195f521c13d4f01