introduce avx_cast<T> to simplify the many different required casts in the AVX impl
introduce avx_cast<T> to simplify the many different required casts in the AVX impl

chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc,VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc