[llvm] [VPlan] Perform interleaving as VPlan-to-VPlan transform. (PR #94339)

[Florian Hahn via llvm-commits]

fhahn wrote:

> Great to see this materialize, and the amount of clean-up it brings!
> 
> Can the patch be split into two main parts:
> 
> 1. introducing the VPlanTransform::interleave() pass, reusing existing code-gen with UF reset to 1
> 2. retiring support for UF > 1 in code-gen, across execute()'s and VPTransformState?
>    plus separating independent clang-format's.
> 

Split off adding the ::interleave part separately as suggested to https://github.com/llvm/llvm-project/pull/95842, which retains the existing structure in the recipes (iterate over parts) and doesn't simplify VPTransformState yet

https://github.com/llvm/llvm-project/pull/94339