[llvm] [VPlan] Narrow VPWidenCastRecipe to scalar cast recipe. (PR #166514)

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon commented:

Hm, maybe not the best idea with the current state of the codebase -- we're planning to eliminate VPWidenCast, and replace it with a VPInstruction, similar to how we handled the scalar cast. Hopefully, that line of work will finally allow us to fix the issue?

https://github.com/llvm/llvm-project/pull/166514