[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?
> 
> This attribute is used to DSE stores _before_ the call. We don't care about what the value after the call is, only about dropping stores before the call.

Ok, thanks, yes, that sounds good.
Writing undef bit if not initialized is a necessary evil because we elide store of undefs :/

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