[llvm] [VPlan] Implement interleaving as VPlan-to-VPlan transform. (PR #95842)

[via llvm-commits]

https://github.com/ayalz commented:

First scan, some comments may be obsolete - drafted before InterleaveState was introduced.

In some cases splitting recipes by dedicating a separate recipe to code generated inside the preheader may simplify interleaving, as the latter need not be cloned across UF.

https://github.com/llvm/llvm-project/pull/95842