[llvm] [LV] Remove common extends and selects in CSE (PR #147731)

[Sam Tebbs via llvm-commits]

SamTebbs33 wrote:

> Does VPlan-based CSE cover most of the cases here? #151872
> 
> If so, I'd prefer to not extend the logic in the legacy pass, but rather work towards removing it. VPlan-based CSE also has the advantage to impact cost estimates

I've had a look at it and the vplan cse pass is run after abstract recipe conversion, so should be able to remove the cloned recipes created by `VPExpressionRecipe`. If it doesn't then I'm happy to add support for whatever is missing.

https://github.com/llvm/llvm-project/pull/147731