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

[Roman Kennke]

Yes, that looks much better. Thank you!

Roman


> 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}	
>