Version Used:
16.5 preview3

Steps to Reproduce:

using System;
public class C {
    public string M1(string obj) {
        return (string)obj;
    }
    public string M2(string obj) {
        return obj as string;
    }
}

Expected Behavior:
The as string in M2 should also be reported as unnessary as it does in M1.
And it does work for Visual Basic
https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA0AbEA3YaAmIA1AD4AKArsBgJZgAEAwhgIYDObTAsAFD3/1K1OvQBiFAHZgALjQgT6AWQCMACgBCATwBqLDPQjAAVvQCCnAMrSoNCQHMAlGcvXbd3gM/0ASjGkUoBUMjDwEAUQl8MUkZOQleUP4hWgZxKVl5JQAmDR09A2NneisbeydzYtd7RK9ff0D6ABUoTUZ2aVVgtErSxxqIqLTY+V4BplYOXiA=

Actual Behavior:
The as string in M2 is not reported as unnessary in source code, but seems compiler already know that and it's included in output IL.