RFR: Fix dangling write-barrier in acmp

[Roman Kennke]

This is needed to fix the recent failure that we have observed with
new-shaped acmp code in jdk8u:

http://cr.openjdk.java.net/~rkennke/fixacmp-jdk11/webrev.00/

The webrev is against sh/jdk11, where the code is also missing (but bug
hasn't showed up in testing). I intend to push the equivalent patch to
sh/jdk8u at the same time.

Testing: hotspot_gc_shenandoah (jdk8u and jdk11), failing specjvm test

Ok?

Roman