Improve formatting within make_mir_visitor macro body.

rustfmt doesn't touch it because it's a macro body, but it's large
enough that the misformatting is annoying. This commit improves it. The
most common problems fixed:

- Unnecessary multi-line patterns reduced to one line.
- Multi-line function headers adjusted so the parameter indentation
  doesn't depend on the length of the function name. (This is Rust code,
  not C.)
- `|` used at the start of lines, not the end.
- More consistent formatting of empty function bodies.
- Overly long lines are broken.

Author: nnethercote