Use esutils.ast.isProblematicIfStatement

I've extracted `esmangle`'s `isProblematicIfStatement` routine to `esutils`.
It covers edge cases. For example,
```js
{
    type: "IfStatement",
    test: EXPR,
    consequent: {
        type: "LabeledStatement",
        body: {type: "IfStatement", test: EXPR, consequent: STMT}
    },
    alternate: STMT
}
```
is also an invalid node.
Oh! I missed it. Right, it's not the test for `isProblematicIfStatement`, it's for `esvalid`.
I'll fix it soon.