Task: Ensure that vec_cast utility is no-op when src and dst types are the same

[oleksandr-pavlyk]

This is a task to ensure that vec_cast<srcT, dstT, N> operator is a no-op if dstT is same as srcT.

[oleksandr-pavlyk]

This will permit code to be faster when sycl::isnan return type changes to return vector of type bool, rather than int, which has already happened in SYCLOS.