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

[Jay Foad via llvm-commits]

jayfoad wrote:

> I agree with @arsenm here [#142359 (comment)](https://github.com/llvm/llvm-project/pull/142359#issuecomment-2939618057)

Me too, and I would happily remove this script when that suggestion is implemented.

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