RFR [8u] 8165489 (S): Missing G1 barrier in Unsafe_GetObjectVolatile

[Aleksey Shipilev]

(trying again on hotspot-dev@)

Hi,

Please review this this backport/fix for 8u. This improves G1 stability in 8u, and provides the base
for Shenandoah JDK 8 backport that piggybacks on G1 pre-barriers. The fix does not apply cleanly,
because there is also Unsafe_GetObject140 that needs fixing too.

JDK 9 bug:
  https://bugs.openjdk.java.net/browse/JDK-8165489

JDK 9 fix:
  http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/a696583f5ddb

JDK 8u fix:
  http://cr.openjdk.java.net/~shade/8165489-8u/webrev.01/

Testing: x86_64 build, Shenandoah tests

Thanks,
-Aleksey