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

[via llvm-commits]

ayalz 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.

Rebasing on top of #93396 will surely reduce the overall size, but enough would remain for such a breakdown to be useful.

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