Fix 1-element vec ambiguities

[GagaLP]

This is the third part of a series of four pull requests addressing some SYCL clarifications for sycl::vec.

This pull request implements the behavior specified in https://github.com/KhronosGroup/SYCL-Docs/pull/670#issue-2715764511, which resolves ambiguities for 1-element sycl::vec.