Rollup merge of #43458 - RalfJung:verbose, r=nikomatsakis

Fix printing regions with -Z verbose

When dumping MIR with `-Z verbose`, it would print regions on types, but not in the code. It seems the Rvalue printing code tried to be smart and guessed when the `Display` for `Region` would not possibly print anything.

This PR makes it no longer be smart, and just always use the `Display` like all the other code (e.g. printing types) does.