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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Latest comments should be addressed. Also moved the unroll code to a separate file.

Also split off the isDefinedOutsideLoopRegions changes to 4eb98384099cbd1d901d8ef6a210af95d9c36adf after landing VPlan-based LICM, which allowed for independent testing.

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