RFR [sh/8u]: Do not enable UseCountedLoopSafepoints in Shenandoah by default

[Roman Kennke]

Enabling -XX:+UseCountedLoopSafepoints in 8u seems risky because it's
known to trigger bugs. In particular, it shows up with the upcoming LRB
backport and the TestMaybeNullUnsafeAccess.java testcase.

Let's not enable it by default, and print a warning when the user
enables it. The downside of it is regressed TTSP for code-paths with
long counted loops.

http://cr.openjdk.java.net/~rkennke/clsp/webrev.00/

Ok?

Roman