This patch updates IRBuilder to create insertelement/shufflevector using poison as a placeholder.
It would be great, but there are many other places that create shufflevector with undef args (InstCombineCalls, etc). Since this and related patches are big changes, I think it might be good to land these first.
If the relevant patches are to be landed at once, I think the series of patches are being too big to maintain... :(
update code comment: undefined -> poison
I'm still catching up on reviews/mails, so ignore if this was already discussed: