RFR: 8339611: GenShen: Simplify ShenandoahOldHeuristics::trigger_collection_if_fragmented [v3]

[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:

  Improve comments and clarify intent of old_region_span

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/492/files
  - new: https://git.openjdk.org/shenandoah/pull/492/files/76bab3ad..327c5009

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=492&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=492&range=01-02

  Stats: 15 lines in 1 file changed: 14 ins; 0 del; 1 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