Description
Translating wgsl with an empty if in the middle of a function generates invalid spirv.

Repro steps
Translate

@workgroup_size(1)
@compute
fn comp(@builtin(global_invocation_id) id: vec3<u32>) {
    if (id.x == 0) {

    }
    _ = 1+1; // otherwise, naga generates returns in the if statement.
    return;
}

into spirv 1.6, then validate it. Demonstrated in https://github.com/Vecvec/wgpu/tree/branch-same-repro.

Expected vs observed behavior
I would expect wgsl that passes validation to also pass spirv validation. This case passes naga validation but not spirv validation

Extra materials

 error: line 25: In SPIR-V 1.6 or later, True Label and False Label must be different labels
  OpBranchConditional %17 %18 %18

Platform
trunk