wgpu: Don't switch on a single constant in output consumed by FXC

glslc emits code like OpSwitch 0 for control flow reasons, for example when one branch ends with a OpKill:

this produces the rather dodgy glsl code:

switch(int(0u)) {
        default:
            ...
            if (((_e46.w * _e49.w) < 0.5)) {
                discard;
            } else {
                break;
            }
            break;
    }
}

On certain GPUs I’ve had this not render the fragment at all (need to double check).

Ideally we should check if there’s only one default case and just not produce the switch in that case.