RFR: 8339611: GenShen: Simplify ShenandoahOldHeuristics::trigger_collection_if_fragmented

[Kelvin Nilsen]

Replace iterative solution with a single mathematical computation, and adjust old-gen defragmentation efforts to be a bit less aggressive.

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Commit messages:
 - Fix whitespace
 - Simplify implementation of trigger_collection_if_fragmented()
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - ... and 14 more: https://git.openjdk.org/shenandoah/compare/662d146d...56479d90

Changes: https://git.openjdk.org/shenandoah/pull/492/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=492&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339611
  Stats: 34 lines in 1 file changed: 4 ins; 10 del; 20 mod
  Patch: https://git.openjdk.org/shenandoah/pull/492.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/492/head:pull/492

PR: https://git.openjdk.org/shenandoah/pull/492