[llvm] [VPlan] Use VPInstructionWithType for uniform casts. (PR #140623)

[Luke Lau via llvm-commits]

lukel97 wrote:

Looks like theres some CI failures: `(GeneratedValue->getType()->isVectorTy() == !GeneratesPerFirstLaneOnly || State.VF.isScalar()) && "scalar value but not only first lane defined"`

https://github.com/llvm/llvm-project/pull/140623