Mangle - Fix label shadowing

[boopathi]

• (Close Error mangling identifier in try-catch when there's same-named label #185)

Not sure if there are any other bugs that this will lead to. If so, afaik, it will only lead to LabelStatement bugs.

• Adds a 250-300ms to mangler because of the double pass. Maybe we can add a flag to useLabels - maybe ? and when babel starts treating Labels in a separate namespace, we can remove the code - incase we merge this...
• this needs traversing program twice because the second time, the vars get replaced from outer scope and we cannot bind the referencedIdentifier to its BindingIdentifier - as we check this based on the Identifier name

[kangax]

I submitted a proper fix so we'll try to get that in first — babel/babel#4758

[boopathi]

Boom !!!