Rollup merge of #137671 - meithecatte:discoverable-dump-mir, r=Nadrieril

Make -Z unpretty=mir suggest -Z dump-mir as well for discoverability

While debugging something else, I got quite annoyed with `-Z unpretty=mir` showing me post-processed MIR instead of the one just after it is built. I ended up asking on Zulip and got pointed to `-Z dump-mir`. While this feature is documented in the rustc dev guide, I think it'd be good if the possibility of making use of it was staring you in the face while you need it.

Author: fmease