Bug 977181 - Don't push/assume IonContext when executing JIT code (r=jandem)

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extra : rebase_source : ffdd773736657d11a67ef4e7f7110f61594d6d1f
Bug 977181 - Rename some ion* things to jit* (r=jandem)

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extra : rebase_source : b407530eb658563f32a5c569d1a0095fc2231d42