[llvm] [VPlan] Scalarize to first-lane-only directly on VPlan (PR #184267)

[Andrei Elovikov via llvm-commits]

https://github.com/eas commented:

Rebase and applied/replied to @artagnon 's comments. 

@fhahn , @artagnon would you take another look at this?

https://github.com/llvm/llvm-project/pull/184267