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

[Reid Kleckner via llvm-commits]

rnk wrote:

This PR actually came up at the LLVM developer meeting round table on AI generated contributions ([notes](https://discourse.llvm.org/t/ai-policy-roundtable-meeting-notes/88718)), and I said this sounds like a totally appropriate use for LLMs. The underlying principle behind the policy has been "how do we ensure that users are not wasting maintainer time by offloading the work of reviewing LLM generated code onto maintainers?"

When it comes to Python utility scripts in the repository, I don't think that maintainers are going through these scripts with a fine-toothed comb to ensure that they pose no threat to the correctness and stability of LLVM compilers, so I think we can say clearly that this is *not* wasting maintainer time and is an encouraged of LLMs.

https://github.com/llvm/llvm-project/pull/164965