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

[Ralf Jung via llvm-commits]

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@@ -27726,15 +27726,18 @@ value.
 Semantics:
 """"""""""
 
-If ``ptr`` is a ``poison`` value, the intrinsic has no effect.
+If ``ptr`` is a ``poison`` value or not a pointer to a stack-allocated object,
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RalfJung wrote:

It seems odd that if I see `%x = poison` I can't make that `%x = 0` as that might cause a validator failure... but you know infinitely more about the actual implementation concerns of this than I do. :)  Just from the theory perspective, it means one cannot prove a general refinement theorem for poison, which certainly throw a wrench into the usual approach of doing this.

But anyway, let's leave this as-is for now; it's not what I originally came here to change anyway.

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