[llvm] [LoopUtils] Fix metadata generated by makeFollowupLoopID (PR #131985)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Looks to me that the mistake is that the `followup_all` contains the LoopID, while it should contain the individual properties. See e.g. https://github.com/llvm/llvm-project/blob/29925b7044517bcd1b3f76333a598092427c3636/llvm/test/Transforms/LoopDistribute/followup.ll#L129-L132 and the documentation at https://llvm.org/docs/TransformMetadata.html.

Sorry, didn't notice that when reviewing the test cast. Whatever created it should be fixed. Whether an MDNode is a LoopID cannot be detected reliably. Having the first entry refer to itself was a technique to avoid collapsing identical LoopIDs before we had `distinct`. There might be other kinds of metadata that also still use this technique. For LoopID it is unnecessary anyway, passes that duplicate code (e.g. inlining, LoopUnroll, ...) do not care about ensuring that each LoopID is distinct anyway. It should be more thought of as a bag of properties.

https://github.com/llvm/llvm-project/pull/131985