Auto merge of #74837 - xldenis:mir-dump-crate-file, r=oli-obk

Auto merge of #74837 - xldenis:mir-dump-crate-file, r=oli-obk

Fix #70767

This PR changes the format of MIR dump filenames to include the crate name rather than `rustc` at the start.

As a result, we can now place mir-opt tests in the same directory as the source files, like with UI tests. I had to make sure that `compiletest` added a bit_width suffix to the expected files when appropriate but otherwise the change is only moving the files to the correct location and ensuring that the `EMIT_MIR` lines are correct.

Fixes #70767
cc @oli-obk