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

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

There aren't so many MIR tests that contain this kind of comment (I counted only 112). Why don't just use `update_mir_test_checks.py`?


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