Auto merge of #116254 - WaffleLapkin:nicen-traversal, r=cjgillot

Assorted improvements for `rustc_middle::mir::traversal`

r? `@cjgillot`

I'm not _entirely_ sure about all changes, although I do like all of them. If you'd like I can drop some commits. Best reviewed on a commit-by-commit basis, I think, since they are fairly isolated.

Author: bors