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

[Ralf Jung via llvm-commits]

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@@ -27734,7 +27721,8 @@ Arguments:
 """"""""""
 
 The argument is either a pointer to an ``alloca`` instruction or a ``poison``
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RalfJung wrote:

I think I realize now that was is meant here is that *in the underlying data structure that represents the IR*, the argument must be such a pointer. That's very confusing since typically, we talk about the IR at th level of an AST, so I thought the "pointer" it refers to here is ``ptr``, and "pointer to alloca instruction" was basically a typo and should say "pointer to stack allocation".

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