Remove from codegen special cases that are supposed to be covered by const-eval

[anton-trunov]

Things like this

if (f.op === "==" || f.op === "!=") {
    if (isNull(f.left) && isNull(f.right)) {
        if (f.op === "==") {
            return "true";
        } else {
            return "false";
        }
...
}