Disallow type predicates as union type members

[weswigham]

function isA<A>(x: any, y: any): x is A | y is B {
    return false;
}

var x: number | number[];
var y: string | string[];

if (isA<number>(x, y)) {
    x, y;
}
else {
    x, y;
}

It makes no sense, but parses correctly, doesn't error (despite the missing identifier), and shows up in quick info as boolean.