Fold comma operation when left argument has no side effects.

(Using https://closure-compiler.appspot.com/home with Advanced Optimizations.)

Currently, the compiler shows a warning:

```
JSC_USELESS_CODE: Suspicious code. This code lacks side-effects. Is there a bug? at line 1 character 13
console.log((1, 0) > 5)
^
```

However, it doesn't (edit:) _always_ constant-fold the comma operator.

This is folded:

console.log((1,0));

This is not:

console.log((1, 0) > 5)

Output:

console.log(5<(1,0));