[llvm] [IA]: Construct (de)interleave4 out of (de)interleave2 (PR #89276)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm commented:

Sorry for the delay.  I've only looked at the interleaving side of things so far but I suspect `GetDeinterleaveLeaves` might have the same issues as `GetInterleaveLeaves` assuming I've not misunderstood the code.

https://github.com/llvm/llvm-project/pull/89276