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

[Valery Pykhtin via llvm-commits]

vpykhtin wrote:

Rebased and squashed.

After some testing I made additional changes:
* Modified code to make cleaner separation between `run_list `and `mir_run_list`.
* Added code and test to check conflicts between ASM and MIR prefixes.
* Added test on update of already updated source (I got double checks at some point).

Check indentation looks consistent with ASM checks.

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