[SYCL] Refactor vector_arith*

This PR separates the "base" case functional implementation from
templates/overloads/constraints. The latter doesn't match even the
current SYCL spec and that is not taking into account proposed
vec/swizzle changes.

Making this change would allow to share more code between
non-preview/preview implementation paths when implementing proposed SYCL
vec/swizzle changes.

Author: aelovikov-intel