[Migrated] Incorrect array stride for arrays of vector types

Issue automatically imported from old repo: https://github.com/EmbarkStudios/rust-gpu/issues/573
Old labels: t: bug,c: rustc_codegen_spirv,s: qptr may fix
Originally creatd by msiglreith on 2021-03-31T18:36:48Z

const QUAD_CLIP: [f32x2; 4] = [
    f32x2 { x: -1.0, y: -1.0 },
    f32x2 { x: 1.0, y: -1.0 },
    f32x2 { x: -1.0, y: 1.0 },
    f32x2 { x: 1.0, y: 1.0 },
];

#[spirv(vertex)]
pub fn quad_vs(#[spirv(vertex_id)] vert_id: i32, #[spirv(position)] a_position: &mut f32x4) {
    let idx = vert_id as usize;
    let pos_clip = QUAD_CLIP[idx];
    *a_position = vec4(pos_clip.x, pos_clip.y, idx as f32, 1.0);
}

QUAD_CLIP type will be generated with an ArrayStride of 8 which is incorrect as in this case all array elements should be treated as vec4 types, requiring a stride of 16.

Generated:

OpDecorate %_arr_v2float_uint_4 ArrayStride 8

Expected:

OpDecorate %_arr_v2float_uint_4 ArrayStride 16

Start with the Rust reproducer for QUAD_CLIP and inspect the generated SPIR-V decoration for its array type. Compare the emitted ArrayStride with the expected value of 16, and consider the issue complete when arrays of vector types receive the correct stride.