MIR-OPT: Pass to deduplicate blocks
This pass finds basic blocks that are completely equal,
and replaces all uses with just one of them.
```bash
$ RUSTC_LOG=rustc_mir::transform::deduplicate_blocks ./x.py build --stage 2 | grep "SUCCESS: Replacing: " > log
...
$ cat log | wc -l
23875
```
15598a83db