RFR: 8321939: [GenShen] ShenandoahOldEvacRatioPercent=100 fails with divide-by-zero [v3]

[Y. Srinivas Ramakrishna]

> The code that used the option ShenandoahOldEvacRatioPercent (SOEP) didn't correctly deal with the extremal setting of 100. Corrected the error and did some light refactoring in the vicinity, getting rid of some dead variables, adding documentation, and assertions. Also modified slightly the documentation of the option.
> 
> **Testing:**
> - [x] GHA
> - [ ] code pipeline testing
> - [x] specjbb with SOEP=default,0,100 to verify that the issue is fixed

Y. Srinivas Ramakrishna has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains ten additional commits since the last revision:

 - Merge branch 'should_unload_classes' into tenure_budget_8314599
 - Merge branch 'master' into should_unload_classes
 - JDK-8322150 should_unload_classes should return true only if a metaspace
   oom has been signalled, and other conditions are met.
 - Merge branch 'master' into tenure_budget_8314599
 - Fix a scope issue with a local variable.
 - jcheck trailing white-space.
 - Merge branch 'master' into tenure_budget_8314599
 - Fix another case where divide-by-zero could occur when
   ShenandoahOldEvacRatioPercent is set to 100. Some slight refactoring in
   the method.
 - Fix divide-by-zero when ShenandoahOldEvacRatioPercent==100; minor
   refactor & modification of documentation of flag

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/369/files
  - new: https://git.openjdk.org/shenandoah/pull/369/files/03e55dee..67318a30

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=369&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=369&range=01-02

  Stats: 88271 lines in 1723 files changed: 41556 ins; 40212 del; 6503 mod
  Patch: https://git.openjdk.org/shenandoah/pull/369.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/369/head:pull/369

PR: https://git.openjdk.org/shenandoah/pull/369