RFR: 8144052: mark_card_deferred does not need to check g1_young_gen

[Erik Helin]

Hi all,

this patch removes an unneeded if statement from
G1SATBCardTableModRefBS::mark_card_deferred. mark_card_deferred is only
called from two places:
- UpdateRSetDeferred::do_oop_nv
- G1ParScanThreadState::update_rs

In the case of UpdateRSetDeferred::do_oop_nv, the check is always false
because we clear the card table in
cleanup_after_oops_into_collection_set_do prior to calling
remove_self_forwarding_pointers.

In the case of G1ParScanThreadState::update_rs, then we can update the
check whether to mark a card or not to check !from->is_young instead of
!from->is_survivor.

Therefore, the if statement:
if (val == g1_young_gen) {
       return false;
}
can be removed from G1SATBCardTableModRefBS::mark_card_deferred.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8144052

Webrev:
http://cr.openjdk.java.net/~ehelin/8144052/webrev.00/

Testing:
- GC test suite with full verification

Thanks,
Erik