Abnormal mutator time for "h2o" and "xalan" with generational plans

[wks]

When testing #849, we found that if the "vo_bit" feature is enabled and the plan is StickyImmix, two benchmarks, namely "h2o" and "xalan", have abnormally high mutator time. See: #849 (comment)

The same phenomenon can be reproduced with GenCopy, but not SemiSpace. See: #849 (comment)

I also tested locally that GenImmix is also affected.

Experiment also shows that the increase in mutator time is not related to heap size. If we increase the heap size, GC will happen less frequently and STW time will decrease, but the mutator time remains the same. See: #849 (comment)

From those evidence, we know that

• It is an existing problem before Fix VO bits for Immix #849
• It affects generational plans, including GenCopy, StickyImmix and GenImmix.