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

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

I would have thought that a VPWidenCastRecipe could have been replicated with its original underlying instruction, but I see now that it's sometimes used for synthetic instructions with no underlying value. We should probably finish moving those over to VPInstructionWithType

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