[8u20] RFR(XS): 8044090: C1: Old value instead of new one is passed to post-barrier in UnsafeGetAndSetObject

[Igor Veresov]

I’d like to backport this small fix, since the problem may manifest itself with nasty and hard to diagnose symptoms.
The patch applies cleanly, nightlies seem clean as well.

Webrev: http://cr.openjdk.java.net/~iveresov/8044090/webrev.00
jdk9 changeset: http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/rev/dd46a4a1299c
JBS: https://bugs.openjdk.java.net/browse/JDK-8044090

Thanks!
igor