Shenandoah Full GC does not preserve age-bits in mark word, so running full GC with string deduplication on, actually dedups nothing.

e.g. running TestStringDedupStress.java with passive mode and disable degenerated GC, will result nothing dedupped.

Since full GC should recover as much memory as possible, we should deduplicate as many string as possible, disregard if their ages reach threshold.