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

[Amanieu d'Antras via llvm-commits]

================
@@ -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,
----------------
Amanieu wrote:

My understanding is that it's a syntactic constraint: the SSA value passed to the intrinsic must have been produced directly by an `alloca` instruction.

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