RFR (sh/11) Proper fix for C2-unrolled-clone with Shenandoah LRB

[Roman Kennke]

Recently I proposed and pushed:

https://hg.openjdk.java.net/shenandoah/jdk11/rev/be804150fdf2

Testing shows that this is not enough. The story around unrolling clone
is fairly complicated pre-jdk12, and the proper fix seems to revert the
above, and properly insert LRBs into unrolled clone loops. It basically
matches what we did pre-LRB by inserting RBs in those places.

http://cr.openjdk.java.net/~rkennke/jdk11-c2-unroll-clone-fix/webrev.00/

Testing: hotspot_gc_shenandoah, specjvm/verifier tests that failed before

Ok?

Roman