RFR [sh/8h]: Fix signatures of ShLRBNode::Value() and ::Identity()

[Roman Kennke]

After the LRB backport, we saw a regression in one of our tests. The
root cause is that ShLRBNode::Value() and ::Identity() have changed
signatures between 8 and 11 and I did not pick that up in 8u backport
and thus those methods never have been called.

Webrev:
http://cr.openjdk.java.net/~rkennke/fixsigs/webrev.00/
Testing:
The failing test passes now. hotspot_gc_shenandoah

Ok to push?

Thanks,
Roman