[llvm] [IR] Add llvm.vector.[de]interleave{4,6,8} (PR #139893)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

It was implemented downstream in a completely separate vectorization framework.  I'm only mentioning it because we don't want to block off the possibility of adding such support in the future.

https://github.com/llvm/llvm-project/pull/139893