RFR: 8295212: G1: Add wrapper for TruncatedSeqs that need separation by gc phase

[Thomas Schatzl]

Hi all,

  can I have reviews for this small refactoring that wraps predictor sequences that are split across gc phases in one more convenient wrapper class to avoid some code repetition?

Testing: tier1

Thanks,
  Thomas

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Commit messages:
 - Cleanup
 - Provide a class that wraps two TruncatedSeqs for young-only/mixed phase

Changes: https://git.openjdk.org/jdk/pull/10793/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10793&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295212
  Stats: 265 lines in 4 files changed: 145 ins; 73 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/10793.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10793/head:pull/10793

PR: https://git.openjdk.org/jdk/pull/10793