[llvm] [LoopDist] Add metadata for checking post process state of distribute… (PR #153902)

[Michael Kruse via llvm-commits]

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@@ -346,6 +346,10 @@ It is recommended to add ``llvm.loop.disable_nonforced`` to
 fallback version (which is likely never executed) is further optimized
 which would increase the code size.
 
+It is recommended to add ``llvm.loop.isdistributed`` to mark loops
+that have been transformed by LoopDistribute so that they are not
+reprocessed under LTO, where they may be given a second opportunity.
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Meinersbur wrote:

The paragraph you copy&pasted is a recommendations what frontends should do. `llvm.loop.isdistributed` is added by LLVM itself. It doesn't make sense to make to recommend frontends to do something they do not have control over.

I had mostly the [Language Reference](https://github.com/llvm/llvm-project/blob/main/llvm/docs/LangRef.rst#L7920) in mind; it is supposed to have an exhaustive list of all recognized metadata.

https://github.com/llvm/llvm-project/pull/153902