[llvm] [VectorCombine] Fold Deinterleave/Interleave Pairs (PR #211022)

[Gaëtan Bossu via llvm-commits]

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gbossu wrote:

Thinking about it, it would help readability to extract two sub-functions from `foldDeinterleaveInterleavePair`, because there clearly are two main steps:
* Collecting all the chains of instructions from a deinterleave back to an interleave (and doing legality checks along the way)
* Processing each level of the chains that were collected and do some kind of SLP vectorisation to make them into a wider single instruction

Super-nit: I think using an `InstVisitor` in each of those functions could help further to really make it clear which instructions are handled.

https://github.com/llvm/llvm-project/pull/211022