RFR: 8339611: GenShen: Simplify ShenandoahOldHeuristics::trigger_collection_if_fragmented [v2]

[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 three additional commits since the last revision:

 - Be less aggressive about triggering old defragmentation
 - Revert "Be even less aggressive in triggering for old fragmentatino"
   
   This reverts commit 8edad5b29b40774073a2a9dc464217341a3e6220.
 - Be even less aggressive in triggering for old fragmentatino

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/492/files
  - new: https://git.openjdk.org/shenandoah/pull/492/files/56479d90..76bab3ad

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=492&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=492&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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