[llvm] Reland "[LV]: Teach LV to recursively (de)interleave." (PR #125094)

[David Sherwood via llvm-commits]

https://github.com/david-arm approved this pull request.

LGTM. I am happy with this approach for now so that you can make progress on this patch. From what I understand, even if you implement VPInstruction::computeCost to properly cost all instructions in VPlan there is still a mismatch between the legacy and VPlan cost model. However, I still think in parallel it does make sense to pursue https://github.com/llvm/llvm-project/pull/125008 with accurate costing for the add VPInstruction.

https://github.com/llvm/llvm-project/pull/125094