RFR: 8339611: GenShen: Simplify ShenandoahOldHeuristics::trigger_collection_if_fragmented [v4]

[Kelvin Nilsen]

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

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

  Respond to reviewer feedback

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/492/files
  - new: https://git.openjdk.org/shenandoah/pull/492/files/327c5009..b9f03090

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=492&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=492&range=02-03

  Stats: 39 lines in 3 files changed: 6 ins; 4 del; 29 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