RFR: Fix satb barrier for object array copy [v4]

[Kelvin Nilsen]

> The satb barrier for overwritten arrays residing in old-gen must be unconditional during young GC.
> 
> During old GC, the satb barrier is only needed for overwritten old-gen arrays if the array is below TAMS within its old-gen region.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Refinements requested by reviewers

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/237/files
  - new: https://git.openjdk.org/shenandoah/pull/237/files/b9c401b1..422372db

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=237&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=237&range=02-03

  Stats: 9 lines in 1 file changed: 4 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/237.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/237/head:pull/237

PR: https://git.openjdk.org/shenandoah/pull/237