[llvm] [LV] Reduce register pressure of RISC-V outer loop reduction. (PR #208621)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> What I mean is, the chain tail of VPExpressionRecipe shouldn't be limited to VPReductionRecipe. We should be able to extend it so that vp.merge can also serve as the chain tail.

Yes absolutely! Currently it is only used for reductions, but it was always intended to be flexible enough to use for any type of idiomatic expression consisting of a set of recipes that should be considered together for cost purposes 

https://github.com/llvm/llvm-project/pull/208621