RFR: 6869327: Add new C2 flag to keep safepoints in counted loops.

[Andreas Eriksson]

Hi,

Please review this change that adds a flag to keep a safepoint in 
counted loops.

Currently C2 removes safepoints in counted loops.
This can force other safepointing threads to wait for the counted loop 
thread for long periods of time.
This change adds a flag, UseCountedLoopSafepoints, which keeps a 
safepoint in the loop. Its default value is false.

Bug: 6869327: Add new C2 flag to keep safepoints in counted loops.
https://bugs.openjdk.java.net/browse/JDK-6869327

Webrev: http://cr.openjdk.java.net/~aeriksso/6869327/webrev.00/

Thanks,
Andreas