No Bug - Fix opt build bustage with generational GC enabled; r=sfink

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extra : rebase_source : 21769149621f7d5219d02d52e78e48d770e1eece
Bug 984684 - Do not fire barriers on shared atoms, round 2; r=sfink

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extra : rebase_source : f27e71470054e61ad424485e4a610bdaa2ff1314