[llvm] [utils] update_llc_test_checks --downstream-target-handler-path option (PR #135879)

[Alexander Richardson via llvm-commits]

arichardson wrote:

If the patches don't change I don't see how this adds a lot of maintainance on your side? I have worked on a fork with huge diff relative to upstream llvm including changes to this script and I think we only needed to resolve one (trivial) conflict over many years.

https://github.com/llvm/llvm-project/pull/135879