When old generation marking is cancelled to run a young collection. we still set a `_degen_point ` for reasons that became vestigial after https://bugs.openjdk.org/browse/JDK-8349094. When old marking is cancelled, the `_degen_point` should only be set if the marking was cancelled because of an allocation failure (and it should still only be set to "outside of cycle"). The following sequence could lead to this assertion failure:
1. Control thread is marking old
2. Young GC preempts it
3. Control thread sets the degen point because the old GC was "cancelled"
4. The concurrent young GC fails and attempts to set a degenerated point
5. This trips the assert because we already (incorrectly) set the degen point in `3`.