[11u] RFR: 8231085: C2/GC: Better GC-interface for expanding clone

[Roman Kennke]

I would like to backport the recent GC interface for expanding clones to
jdk11u. This is a prerequisite to backport related Shenandoah changes to
11u without making a mess.

The change differs from the original jdk14 change because it basically
skips the intermediate GC interface for the same thing that's been
introduced in jdk12. This one wholly replaces that.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8231085
Original webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8231085/webrev.00/

JDK11u webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8231085/webrev.00.jdk11u/

Testing: tier1 and tier2 no regressions

Good?

Roman