Rollup merge of #155641 - Zalathar:no-run-mir-opt, r=jieyouxu

Remove non-working code for "running" mir-opt tests

Tests in `tests/mir-opt` always use `--emit=mir`, so the compiler doesn't even produce an executable.

Attempting to "run" these tests (e.g. with `./x test mir-opt --pass=run`) therefore fails when the OS notices that a MIR text file is not executable.

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The second commit performs some semi-related cleanup.

r? jieyouxu