Improve already-present performance in computeIfAbsent, putIfAbsent

[squallliu]

https://bugs.java.com/bugdatabase/view_bug.do?bug_id=8161372

http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/ConcurrentHashMap.java?r1=1.295&r2=1.296

[karianna]

Backport requests should be made upstream at openjdk.java.net (search for the jdk8u mailing list!).