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

[Gaëtan Bossu via llvm-commits]

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gbossu wrote:

Yeah I'm not entirely sure it would help, that's why I marked that as super-nit 😄

I thought we could still benefit from all the dispatching by using `visitInst` only. But what would be great would be to introduce a `visitInstLevel(ArrayRef<Instruction> Insts)` method that would automatically build up `SmallVector<ElementwiseStep, 4> Steps` as we progress through the levels.

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