RFR: 8295149: Misnomer for_young_gc instead of for_young_only_phase in G1Analytics

[Thomas Schatzl]

Hi all,

  please review this change that renames lots of `for_young_gc` parameters to `for_young_only_phase` to correspond to the actual meaning.

Depends on PR#10650, so please review that one first.

Testing: local compilation

Thanks,
  Thomas

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Depends on: https://git.openjdk.org/jdk/pull/10650

Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/10651/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10651&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295149
  Stats: 51 lines in 7 files changed: 0 ins; 0 del; 51 mod
  Patch: https://git.openjdk.org/jdk/pull/10651.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10651/head:pull/10651

PR: https://git.openjdk.org/jdk/pull/10651