RFR: merge of 8203197

[Roland Westrelin]

The upstream change merged with the current shenandoah repo:

http://cr.openjdk.java.net/~roland/shenandoah/backport-8203197/webrev.00/

Some fixes to the upstream changes that I found are required for
shenandoah:

http://cr.openjdk.java.net/~roland/shenandoah/fixes-8203197/webrev.00/

And a collection of small fixes unrelated to the upstream but I found
while testing the change:

http://cr.openjdk.java.net/~roland/shenandoah/various-c2-fixes/webrev.00/

Roland.