"Raises an invalid opcode exception in all operating modes."
What is here undefined? LLVM must not generate such instructions except it it really wants such a exception. (Like Linux's panic() does on x86)
I'm not so sure about that. If you need to squeeze out a little more performance, code that is "technically undefined" can be more portable than dropping to ASM.
I think LLVM should emit a warning on code with undefined semantics and generate DWIM instructions instead of UD2s.
But you're correct that sometimes it can be expedient to exploit such technically undefined behavior. (I've committed this sin myself, most commonly in serializers/deserializers)