RFR: Disable redundant generation usage after fullgc [v2]

[Kelvin Nilsen]

> After a recent integration, we ended up recomputing generation usage twice.  This change integrates the two efforts with a single more efficient pass.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Remove irrelevant changes to shenandoahReferenceProcessor.cpp
  
  These changes were made to pass regression testing.  See PR#241.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/242/files
  - new: https://git.openjdk.org/shenandoah/pull/242/files/d93a94b1..534da0ff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=242&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=242&range=00-01

  Stats: 10 lines in 1 file changed: 0 ins; 7 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/242.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/242/head:pull/242

PR: https://git.openjdk.org/shenandoah/pull/242