[llvm] [SimplifyCFG] Add loop metadata to the new branch when doing jump-threading (PR #157180)

[Nikita Popov via llvm-commits]

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nikic wrote:

Please use update_test_checks.py so we can see the full IR. Please also use named values in the test (like `loop` instead of `147`).

https://github.com/llvm/llvm-project/pull/157180