Bug 911578 - Introduce self-hosting intrinsic isPackedArray. r=jandem

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extra : rebase_source : de128118f4a52553efa77f70bb4699dfb94d97c4
Bug 911578 - Use self-hosting intrinsic isPackedArray to optimize loops in array extras. r=jandem

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extra : rebase_source : 1b01fa6136f2bac70e98f8e8e900deb08d9dd3b8