Enable blessing of mir opt tests
cc @rust-lang/wg-mir-opt
cc @RalfJung
Long overdue, but now you can finally just add a
```rust
// EMIT_MIR rustc.function_name.MirPassName.before.mir
```
(or `after.mir` since most of the time you want to know the MIR after a pass). A `--bless` invocation will automatically create the files for you.
I suggest we do this for all mir opt tests that have all of the MIR in their source anyway
If you use `rustc.function.MirPass.diff` you only get the diff that the MIR pass causes on the MIR.
Fixes#67865
0a2df62073