[8] RFR: Rectify JDK-8191227 workaround for Shenandoah

[Aleksey Shipilev]

When we were developing 10, we came across the Trouble, and worked that around:
  https://mail.openjdk.java.net/pipermail/shenandoah-dev/2017-November/004296.html

With LRB in 8u and other fixes, this does not directly affect Shenandoah anymore. However, touching
naked oops is still dangerous, and we would want to backport the whole change to 8u upstream:
  https://bugs.openjdk.java.net/browse/JDK-8191227

We have reverted the workaround in 11+ already in favor of JDK-8191227:
  https://hg.openjdk.java.net/shenandoah/jdk11/rev/d82146d195fc

Until that the backport happens in 8u, I think we want to reshape the workaround to do only the
necessary things like JDK-8191227 does it. For example, I believe entering VM there is unnecessary
and even harmful (for example for JVMCI entering there already in VM state!) for product builds.
JDK-8191227 has some bugtail, I don't want to cherry-pick that directly.

8u webrev:
  https://cr.openjdk.java.net/~shade/shenandoah/8u-rectify-debugInfo/webrev.01/

Full diff against 8u would be:
  https://cr.openjdk.java.net/~shade/shenandoah/8u-rectify-debugInfo/8u-upstream.diff

Testing: hotspot_gc_shenandoah {fastdebug,release}	

-- 
Thanks,
-Aleksey