[llvm] [VPlan] Impl VPlan-based pattern match for ExtendedRed and MulAccRed (NFCI) (PR #113903)

[Florian Hahn via llvm-commits]

fhahn wrote:

> The way I think I had imagined this working in a vplan-based cost model would be that the vplan nodes more closely mirrored what the back-end would produce. So there would be a vplan recipe for the extended-reduction, which would be created if it was profitable but would otherwise be relatively easy to cost-model.
> 

Yes I think ideally we would model the add-extend/mul-extend operations explicitly in VPlan, especially if matching it in VPlan would require changes to the order in which the cost is computed. Would add-extend/mul-extend be sufficient or would other recipes be needed as well?

https://github.com/llvm/llvm-project/pull/113903