This commit extracts some MIR-related code from `common.py` and
`update_mir_test_checks.py` into a dedicated `mir.py` module to improve
code organization. This is a preparation step for
https://github.com/llvm/llvm-project/pull/164965 and also moves some
pieces already moved by https://github.com/llvm/llvm-project/pull/140296
All code intentionally moved verbatim with minimal necessary
adaptations:
* `log()` calls converted to `print(..., file=sys.stderr)` at `mir.py`
lines 62, 64 due to a `log` locality.
44f5ae3eec