Add some missing UseShenandoahGC checks to 8u

[Roland Westrelin]

> I see! I wonder if it's cleaner/safer to revert to upstream and
> conditionalize the entire block on UseShenandoahGC then, to make it
> obvious the upstream code stays the same, instead of trying to play
> games with locals... I.e. so that the original upstream code was the
> same "if" branch:

This?

http://cr.openjdk.java.net/~roland/shenandoah/8u_useshenandoahgc/webrev.01/

Roland.