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

[David Green via llvm-commits]

davemgreen 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.

I'm not sure if that is still the current plan or not.

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