avoid cycles in mir-dump, take 2 #42017
                
     Merged
            
            
          
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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