Fix #905: Show methods the optimizer worked on if it crashes

The error message will look like this:

    The Scala.js optimizer crashed while optimizing sjs_test_jsinterop_UndefOrTest$$anonfun$1$$anonfun$apply$mcV$sp$7.init___sjs_test_jsinterop_UndefOrTest$$anonfun$1
    Methods attempted to inline:
    * sr_AbstractFunction0$mcV$sp.init___
    * sr_AbstractFunction0.init___
    * O.init___
This will add a same `target` multiple times if the same method impl is inlined (or tried to) several times. Either `attemptedInlining` should be a set, or this should avoid duplicates, or you should call `.distinct` when converting to a `List`. The latter's the probably the most efficient, since converting to a `List` is, in fact, never supposed to happen (since it is done only in the event of crash).