RFR: 8342560: GenShen: Fix confusing method name [v3]

[William Kemper]

> Rename `is_old` to make it more clear that this about pointers that are outside the scope of a young collection.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 10 additional commits since the last revision:

 - Merge remote-tracking branch 'shenandoah/master' into fix-confusing-method
 - Use an even better name, add an explanatory comment
 - 8342564: GenShen: Only reference young/old generation names in generational mode
   
   Reviewed-by: xpeng, ysr
 - 8342214: GenShen: Reduce code duplication in shFreeSet with iterator abstraction
   
   Reviewed-by: kdnilsen, ysr
 - 8342239: GenShen: Revert changes in adaptive heuristic to avoid overflow on 32 bit
   
   Reviewed-by: ysr
 - 8342278: GenShen: Move non-generational mode test out of generational test configuration
   
   Reviewed-by: ysr
 - 8342255: GenShen: Remove unnecessary enum initial values
   
   Reviewed-by: ysr
 - Fix confusing method name
 - Revert "Test if we still (or ever) needed confusing is_old method"
   
   This reverts commit 5fcaef551d3286bd3e5eae485ac11c5b15ce7ed8.
 - Test if we still (or ever) needed confusing is_old method

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/517/files
  - new: https://git.openjdk.org/shenandoah/pull/517/files/9bb4dfb7..90c81bdb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=517&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=517&range=01-02

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/517.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/517/head:pull/517

PR: https://git.openjdk.org/shenandoah/pull/517