[LangRef][Local] dereferenceable metadata violation is UB

I believe !dereferencable violation is immediate undefined behavior,
but this was not explicitly spelled out in LangRef. We already
assume that !dereferenceable is implicitly !noundef and cannot
return poison in isGuaranteedNotToBeUndefOrPoison().

The reason why we made dereferenceable implicitly noundef is that
the purpose of this metadata is to allow speculation, and that
would not be legal on a potential poison pointer.

Differential Revision: https://reviews.llvm.org/D148202