RFR: 8295212: G1: Add wrapper for TruncatedSeqs that need separation by gc phase [v3]

[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

Thomas Schatzl has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge branch 'master' of https://git.openjdk.org/jdk into submit/8295212-wrapper-for-truncatedseqs2
 - Use correct sequence for checking mixed samples
 - Cleanup
 - Provide a class that wraps two TruncatedSeqs for young-only/mixed phase

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Changes: https://git.openjdk.org/jdk/pull/10793/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10793&range=02
  Stats: 260 lines in 4 files changed: 145 ins; 68 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