RFR (XS): CR 6914402: G1: assert(!is_young_card(cached_ptr), "shouldn't get a card in young region")

[John Cuthbertson]

Hi Everyone,

Can I have a couple of volunteers review the changes for this CR? The 
webrev can be found at: http://cr.openjdk.java.net/~johnc/6914402/webrev.0/.

The issue here was that a clean up pause had freed a region which had 
all garbage which had cards in hot card count cache. After this region 
had been reallocated as a young region, a subsequent insertion into the 
card count cache evicted the card in the now-young region causing the 
assert to trip. This assert was one that I had actually added in the fix 
for 6819077.

Testing: GCBasher (30+ hours), jprt.

Thanks,

JohnC