7129892: G1: explicit marking cycle initiation might fail to initiate a marking cycle Summary: If we try to schedule an initial-mark GC in order to explicit start a conc mark cycle and it gets pre-empted by antoher GC, we should retry the attempt as long as it's appropriate for the GC cause. Reviewed-by: brutisso, johnc

7144296: PS: Optimize nmethods processing Summary: Prunes scavenge roots in code list every young GC, promote objects directly pointed by the code immediately Reviewed-by: johnc, jcoomes

Merge