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

[via llvm-commits]

vporpo wrote:

Thanks for your feedback @arichardson . The main argument is along the lines of what @rnk mentioned above. The patch provides a way to decouple downstream from upstream and make the `update_llc_test_checks.py` script target unaware, which I think is overall considered desirable from the design and maintenance perspective. I agree that maintaining the downstream patches won't probably be an issue as conflicts are unlikely. But since there is a way to avoid the patches in the first place I am arguing that this should be the preferred solution.

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