Function select_impl 

pub fn select_impl(policy: &KernelPolicy) -> KernelImpl

Resolve the implementation for a batch from policy.

Selection order:

1. force_scalar, or neither portable nor arch allowed → KernelImpl::Scalar
2. allow_arch_simd and compiled simd-runtime and CPU support → KernelImpl::ArchSimd
3. allow_portable_optimized → KernelImpl::PortableOptimized
4. else → KernelImpl::Scalar