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

[Paul Walker via llvm-commits]

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

I've not looked at the ISelLowering changes yet because I want to get an understanding of what we need to match first because that might dictate where the code ends up.

The patch could also do with some SVE VLS tests (see fixed-deinterleave-intrinsics.ll).  I'm not asking for this work to support such types, only that tests exist to ensure they don't trigger any failure paths.  This mirrors the same request I had for https://reviews.llvm.org/D146218.

Preferable the tests would be the first commit on this PR with the transformation in following commits so that it's easy to see the effect of the new code.  I guess you could precommit the tests but personally I'm fine with them remaining within this PR.



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