Optimize oop scan closure closures wrt to reference processing in G1

G1 oop scan closures inherit from G1ScanClosureBase that sets the java.lang.ref.reference reference_iteration_mode to DO_DISCOVERED_AND_DISCOVERY.

However most of these closures always have a NULL reference processing closure, so there is no point in trying to do discovery. While the impact is only an additional NULL check for InstanceRefKlass instances, it is unnecessary.

Look through G1 oop scan closures and set the correct reference_iteration_mode for them (i.e. DO_FIELDS).