[ET-VK] Fake u16vecn for devserver (#6704)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6675

## Context

Copy-pasted from the newly added `maybe_fake_u16vec3` function in the codegen script:

> There is a latency benefit to using u16vecn variables to store texture position variables instead of ivecn, likely due to reduced register pressure. However, SwiftShader does not support 16 bit integer types in shaders, so this is a crude way to fallback to using ivecn to store texture positions so that testing with SwiftShader is still possible.


ghstack-source-id: 252234981
@exported-using-ghexport

Differential Revision: [D65501674](https://our.internmc.facebook.com/intern/diff/D65501674/)

Co-authored-by: Stephen Jia <ssjia@meta.com>