[wgsl-in] Only splat 'scalar op vec', not '_ op vec'

[fornwall]

Change binary_op_splat() from splatting:

    vec op scalar
    _ op vec

to only splat:

    vec op scalar
    scalar op vec

This fixes some confusing error messages. Example function:

fn main(mat: mat3x3<f32>) {
    let vec = vec3<f32>(1.0, 1.0, 1.0);
    let result = mat / vec;
}

[fornwall]

Error message before this change (note the Scalar type Matrix message):

[fornwall]

Error message after this change (in general formatting of validation errors needs improvement, but the underlying information is now correct):

[teoxoy]

Makes sense, thanks!