RFR(S): 8235636: gc/shenandoah/compiler/TestUnsafeOffheapSwap.java fails after JDK-8226411

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8235636/webrev.00/

ShenandoahBarrierC2Support::is_dominator_same_ctrl() must take
anti-dependencies into account otherwise when updating raw memory after
a barrier is expanded, a load may be reordered wrt a store.

Roland.