[LangRef] insertelement/extractelement return poison for out of range.

We need to clarify the language here. I think poison makes more sense
than undef, since it's an undefined operation rather than uninitialized
memory. I don't think anything depends on the difference at the moment,
though.

Differential Revision: https://reviews.llvm.org/D47859

llvm-svn: 334325