[llvm] [LV][VPlan] Prevent calculate cost for skiped instructions in precomputeCosts(). (PR #127966)

[Florian Hahn via llvm-commits]

fhahn wrote:

I think it would be good to pick this on the 20.x release branch. I requested a back port: https://github.com/llvm/llvm-project/pull/128694

https://github.com/llvm/llvm-project/pull/127966