[llvm] update_test_checks: improve IR value name stability (PR #110940)

[Nicolai Hähnle via llvm-commits]

nhaehnle wrote:

> > By default, UTC attempts to keep the produced diff small by keeping IR value name variables stable.
> 
> FWIW I don't think this functionality actually works in practice. I get the same amount of "rename diffs" before and after this feature was introduced. Does it only work for some specific UTC `--version` and not any of the older tests, or something like that?

It really should work independent of the version and it has worked for me -- though clearly with some weaknesses which this change is meant to address :)

The way to turn it off is via the --reset-variable-names option.

https://github.com/llvm/llvm-project/pull/110940