Fix magn 5300: ScopeIf is broken 

Fix defect [MAGN 5300: ScopeIf is broken] (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5300#)

ScopeIF's preview value is always "null". The root cause is the variable name that is used to query preview value for ScpeIf node is different from the variable that is assigned the value. 

Unifying them by using ``AstIdentifierForePreview`` property. 

@Benglin  please review the change. 
public or protected?

Is there a need to make this part of the public API?