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

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

I think this makes sense to me. I just want to check how will this be extended in future? Will we eventually also scalarize uniform instructions here too so that `VPRecipeBuilder::handleReplication` only emits non isSingleScalar replicate recipes?

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