G1 should not clear mark bitmaps with no marks

Currently G1 clears the whole (next) mark bitmaps of the regions of the entire (committed) heap.

This is unnecessary, only bitmaps of regions that were dirtied need clearing. Since JDK-8197850 we track how much live data has been encountered during mark for every region, corresponding to the live objects we set a mark between bottom and ntams.

I.e. first, we only need to clear between [bottom, ntams[, and second only clear bitmaps which regions had at least one bit set (i.e. have some live data).

This is also a very interesting optimization for JDK-8240556