[llvm] [utils] Add a script to update regclass numbers in MIR tests (PR #142761)

[Shilei Tian via llvm-commits]

https://github.com/shiltian commented:

I'm not really familiar with our rules regarding implementing utils. Do we need it to be in Python for portability? Other than that, this looks good to me.

https://github.com/llvm/llvm-project/pull/142761