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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

If the legacy-cost estimates are worse, I don't think we need to try hard to match them, if we can make use of more accurate modeling in VPlan. If that's the case, I'd use `planContainsAdditionalSimplifications` to allow divergences between legacy and VPlan-based cost model for plans that contain extended reduction recipes.

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