RFR: Bulk backports to sh/jdk11 2020-04-20

[Roman Kennke]

This backports the following changes from jdk/jdk:

  8242316: Shenandoah: Turn NULL-check into assert in SATB slow-path entry
  8242301: Shenandoah: Inline LRB runtime call
  8242273: Shenandoah: accept either SATB or IU barriers, but not both
  8242271: Shenandoah: add test to verify GC mode unlock
  8242054: Shenandoah: New incremental-update mode
  8242217: Shenandoah: Enable GC mode to be diagnostic/experimental and
have a name
  8242130: Shenandoah: Simplify arraycopy-barrier dispatching
  8241700: Shenandoah: Fold ShenandoahKeepAliveBarrier flag into
ShenandoahSATBBarrier
  8241675: Shenandoah: assert(n->outcnt() > 0) at
shenandoahSupport.cpp:2858 with
java/util/Collections/FindSubList.java


http://cr.openjdk.java.net/~rkennke/backport-shjdk11-2020-04-20/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Roman