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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> By representing these higher factors are interleaved-interleaves, we can in theory support arbitrarily high interleave factors. However I'm not sure this is actually needed in practice.

You can definitely end up with very large interleave factors in some cases; my team has internal testcases for stride 24.  Granted, it's uncommon.

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