RFR: 8342560: GenShen: Fix confusing method name [v2]

[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 incrementally with six additional commits since the last revision:

 - 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

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/517/files
  - new: https://git.openjdk.org/shenandoah/pull/517/files/da36b0a2..9bb4dfb7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=517&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=517&range=00-01

  Stats: 229 lines in 9 files changed: 93 ins; 69 del; 67 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