Do a better job of pruning dead code

[benjamn]

Tools like UglifyJS can do a lot of this, but we should try harder upstream, too.

We already attempt to prune obviously unreachable code in Emitter.prototype.getDispatchLoop (see isSwitchCaseEnder and alreadyEnded), but I disabled that temporarily because that extra cleverness made it harder to verify the generated code.

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