RFR: 8270041: Consolidate oopDesc::cas_forward_to() and oopDesc::forward_to_atomic()

[Roman Kennke]

The two methods do exactly the same, except one returns a bool, the other a value. The only use of cas_forward_to() can be consolidated to use forward_to_atomic() instead.

Testing:
 - [ ] hotspot_gc
 - [ ] tier1
 - [ ] tier2

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Commit messages:
 - 8270041: Consolidate oopDesc::cas_forward_to() and oopDesc::forward_to_atomic()

Changes: https://git.openjdk.java.net/jdk/pull/5167/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5167&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270041
  Stats: 10 lines in 3 files changed: 0 ins; 9 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5167.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5167/head:pull/5167

PR: https://git.openjdk.java.net/jdk/pull/5167