RFR: 8339960: Shenandoah: Fix inconsistencies in generational Shenandoah behaviors

[Kelvin Nilsen]

This fixes some bugs found in recent code review and playback of an assertion failure.

See also https://github.com/openjdk/shenandoah/pull/497

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Commit messages:
 - Use -1 for rightmost interval when range is empty
 - Check available rather than capacity before logging shortfall
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Revert "Make GC logging less verbose"
 - Make GC logging less verbose
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - ... and 13 more: https://git.openjdk.org/jdk/compare/81ff91ef...f1ba63f4

Changes: https://git.openjdk.org/jdk/pull/20974/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20974&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339960
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20974.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20974/head:pull/20974

PR: https://git.openjdk.org/jdk/pull/20974