[llvm] [LV] Support EVL for partial reduction and VPExpressionRecipe. (PR #205741)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Gentle ping! Could @fhahn and @lukel97 help clarify the preferred direction here? We have conflicting suggestions on whether to implement EVL transformations for ExpressionRecipes in VPlanTransforms or VPExpressionRecipe. I am happy to implement either approach.

My personal philosophy is that transforms and vectorization strategies should be as standalone as possible. I.e. recipes and transforms shouldn't need to know about EVL if they don't have to, to maintain separation of concerns, improve composability etc. 

Hence why I think calling decompose and leaving the EVL specific transform logic out of VPExpressionRecipe is more maintainable. I think the rest of the patch looks pretty good to me though, so I wouldn't block it if others feel very strongly the other way.

https://github.com/llvm/llvm-project/pull/205741