[llvm] [IR] LangRef: document behavior of double-lifetime.start (PR #184296)

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

This reverts LangRef back to the previous version.
I'm not a big fan of this semantics because it's not useful (thus makes Alive2's encoding more expensive for no good reason). We don't have any use case for this right now. Right now these intrinsics are introduced by the inliner and used by stack coloring. Lifetime start can never show up twice.

The use case that was discussed before was to cut loop-carried dependencies, but nothing was ever implemented, and thus it's unclear it would actually help.

I would rather have it UB.

https://github.com/llvm/llvm-project/pull/184296