This prepares for having other vector sizes (e.g. byte8) support member
methods such as broadcast assignment.

Note the partial specialization for N=4, to have x,y,z,w members.

Bug: b/146224130
Change-Id: I09a341812ddfa14d247e94fd3f9345db951824ed
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39554
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>