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

[Paul Walker via llvm-commits]

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

This time I've limited myself to the deinterleave side of things.  The code structure now looks very good with my comments aiming to simplify a few things whilst bringing the existing Factor==2 code in line with your Factor==4 implementation that's much more resilient.

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