RFR: 8260736: Shenandoah: Cleanup includes in ShenandoahGC and families [v2]

[Zhengyu Gu]

> 8260736: Shenandoah: Cleanup includes in ShenandoahGC and families

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Added back vmThread.hpp

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2339/files
  - new: https://git.openjdk.java.net/jdk/pull/2339/files/001c3094..87924b4c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2339&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2339&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2339.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2339/head:pull/2339

PR: https://git.openjdk.java.net/jdk/pull/2339