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

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vporpo wrote:

Thank you for your reply @MaskRay . These scripts are indeed stable, so they won't probably cause any merge conflicts. But there is a downstream cost for each one of these patches as they have to go through a process for getting them added in or getting them updated, which could be saved if we added this simple feature.

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