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

[Andrei Elovikov via llvm-commits]

eas wrote:

> 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?

Ideally we'll just port all scalarization logic from legacy CM to here someday. At the moment I'm *not* working on that as my priority is to port unit-stride speculation. I'd expect next step after that might be supporting interleave groups with RT stride (`vlsseg*`) and porting interleave groups logic to be done directly on VPlan. At that point we'd move their creation to before this pass (and, hopefully, old widening logic won't be used for address calculation chain anymore after that).

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