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

[Florian Hahn via llvm-commits]

fhahn wrote:

> Just out of interest @fhahn do you have an idea of when this assert can be removed?

There are still a number of cost computation that need to be updated, for those the assert is still valuable I think (as this example also highlights, as it surfaced a case where the VPlan-based cost model missed the cost of some instructions).

Now that we branched for clang-20 I'll look into finishing the transition without risking introducing additional instability on the clang-20 release 

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