Bug 777474 - Use the expression decompiler for opcode text when possible. r=bhackket

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extra : rebase_source : 84a1421dfdf12c8e6fd2bb8a09132cd83c13c590
Bug 718969 - Perform an exorcism; remove the decompiler. r=luke

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extra : rebase_source : 64df8f9a7ac230e1462a8f6e134ac3e86258d6f8