RFR: 8370726: GenShen: Misplaced assertion that old referent is marked during young collection

[William Kemper]

Recently introduced assertion is too strong:

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/codebuild/output/src2096/src/s3/00/src/hotspot/share/gc/shenandoah/shenandoahReferenceProcessor.cpp:332), pid=1373, tid=1375
# Error: Marking should be complete for object 0x00000007ffc029b0 in the Old generation

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

Commit messages:
 - Something in full gc still needs 'active' generation?
 - Give reference processor a generation field

Changes: https://git.openjdk.org/jdk/pull/28026/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28026&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370726
  Stats: 27 lines in 3 files changed: 4 ins; 14 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/28026.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28026/head:pull/28026

PR: https://git.openjdk.org/jdk/pull/28026