RFR: 8233439 : G1 zero_filled optimization when committing CardCountsTable does not work

[stefan.johansson at oracle.com]

Hi,

Please review this fix to clear memory correctly when committing regions.

Webrev: http://cr.openjdk.java.net/~sjohanss/8233439/00/
Issue: https://bugs.openjdk.java.net/browse/JDK-8233439

Summary
The old code always set all_zero_filled to false if more than one region 
belonging to the same page was committed. This leads to unnecessary 
clearing of memory in some data structures.

While fixing all_zero_filled I also cleaned up the code a bit more to 
get rid of the refcount array and instead use the commit bitmap to check 
if pages are already committed.

Testing
Local testing looks good, currently running mach5 1-3. Local tests also 
show a slight improvement in startup.

Thanks,
Stefan