JDK-6991197 : G1: specialize deal_with_reference() for narrowOop*
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G1ParScanThreadState::deal_with_reference(T*) should be specialized for narrowOop*; doing so may help performance when compressed oops are enabled.
deal_with_reference(T*) first checks whether the partial array mask bit is set and branches accordingly. This check is unnecessary when T is narrowOop, so the test and branch can be eliminated for narrowOop* (presumably the most common type processed) if the method is specialized.