RFR: 8342560: GenShen: Fix confusing method name

[William Kemper]

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

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Commit messages:
 - Fix confusing method name
 - Revert "Test if we still (or ever) needed confusing is_old method"
 - Test if we still (or ever) needed confusing is_old method

Changes: https://git.openjdk.org/shenandoah/pull/517/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=517&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342560
  Stats: 7 lines in 3 files changed: 0 ins; 0 del; 7 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