[llvm] Reapply "[utils][UpdateLLCTestChecks] Add MIR support to update_llc_test_checks.py." (#164965) (PR #166575)

[David Spickett via llvm-commits]

https://github.com/DavidSpickett approved this pull request.

Can confirm that `x86-asm-mir-same-prefix.test` was the only failing test previously, so this LGTM.

https://github.com/llvm/llvm-project/pull/166575