RFR: jdk11: guard shared code with UseShenandoahGC

[Roland Westrelin]

>> I also propose we drop the changes related to the register doubling stuff from 11.
>
> Not sure, why? Can we "just" protect them with UseShenandoahGC as well?

In previous releases, we dropped mid/large size shared code changes that
were not upstream and not required. That one hasn't proven to bring any
benefit either, it could most likely be guarded by UseShenandoahGC but
it just seems simpler/safer to remove it.

Roland.