[llvm] [LangRef] Update initializes definition (PR #134370)

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

On higher-level question: is this attribute useful as specified?
The caller only knows that the initialized range was written to, but it can be poison, undef, or a concrete value. There's nothing it can assume about the stored value. All it knows it that the range is dereferenceable, but for that we already have an attribute.
What am I missing?

https://github.com/llvm/llvm-project/pull/134370