Mir pretty print: Add cleanup comment

[diwic]

I found it useful to add a comment indicating whether or not a
BasicBlock is a cleanup block or not. Hopefully you'll find it
useful too.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @arielb1 (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[diwic]

Example:

bb1: {                              
    _2 = const middle() -> [return: bb2, unwind: bb4]; // scope 1 at /tmp/main.rs:7:2: 7:10
}

bb2: {                              
    _0 = ();                         // scope 0 at /tmp/main.rs:5:11: 8:2
    drop(_1) -> bb5;                 // scope 0 at /tmp/main.rs:8:2: 8:2
}

bb3: {                               // cleanup
    resume;                          // scope 0 at /tmp/main.rs:5:1: 8:2
}

bb4: {                               // cleanup
    drop(_1) -> bb3;                 // scope 0 at /tmp/main.rs:8:2: 8:2
}

bb5: {                              
    StorageDead(_1);                 // scope 0 at /tmp/main.rs:8:2: 8:2
    return;                          // scope 0 at /tmp/main.rs:8:2: 8:2
}

[arielb1]

@bors r+

[bors]

📌 Commit 6f8b69f has been approved by arielb1

[arielb1]

@bors rollup

[bors]

⌛ Testing commit 6f8b69f with merge f1b5225...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: arielb1
Pushing f1b5225 to master...