RFR: 8266083: Shenandoah: Consolidate dedup/no dedup oop closures

[Zhengyu Gu]

We can use template to consolidate dedup/no dedup oop closures.

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Commit messages:
 - update
 - init

Changes: https://git.openjdk.java.net/jdk/pull/3724/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3724&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266083
  Stats: 90 lines in 3 files changed: 13 ins; 53 del; 24 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3724.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3724/head:pull/3724

PR: https://git.openjdk.java.net/jdk/pull/3724