[llvm] [VPlan] Implement VPExtendedReduction, VPMulAccumulateReductionRecipe and correspond vplan transformations. (PR #137746)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm approved this pull request.

Given that this is just code split out from the already reviewed PR #113903, this looks good to me (with @huntergr-arm's comment addressed)

One minor nit on the commit message:
> and correspond vplan transformations

->

> and corresponding vplan transformations


https://github.com/llvm/llvm-project/pull/137746