RFR: 8262793: Shenandoah: Need to restore reference marking strength after mark through subgraph

[Zhengyu Gu]

During marking phase, wavefront may switch to weak marking, but need to restore back to original wavefront after mark through the subgraph.

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Commit messages:
 - JDK-8262793

Changes: https://git.openjdk.java.net/jdk/pull/2784/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2784&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262793
  Stats: 22 lines in 3 files changed: 17 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2784.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2784/head:pull/2784

PR: https://git.openjdk.java.net/jdk/pull/2784