RFR: 8194406: Use Atomic::replace_if_null

[Kim Barrett]

> On Jan 3, 2018, at 3:15 AM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
>> On Jan 2, 2018, at 11:41 PM, David Holmes <david.holmes at oracle.com> wrote:
> 
>> ./share/utilities/bitMap.cpp:    if (Atomic::cmpxchg(table, &_pop_count_table, (BitMap::idx_t*)NULL) != NULL) {
> 
> Thanks for spotting that one.

I missed that one because my grep pattern didn't include qualified
type names in the cast.  Fixing my search, I didn't find any other
calls to update.

Here's the updated webrev:
http://cr.openjdk.java.net/~kbarrett/8194406/open.01/

Only change is to bitMap.cpp.