Rollup merge of #42017 - nikomatsakis:issue-41677, r=arielb1

avoid cycles in mir-dump, take 2

This fixes #41697, for real this time, but I'm not sure how best to add a regression test. I was considering maybe adding some flag so that the MIR dumping doesn't actually get written to files (e.g., overloading the directory flag so you can specify nil or something).

cc @dwrensha @oli-obk