RFR: 8321939: [GenShen] ShenandoahOldEvacRatioPercent=100 fails with divide-by-zero [v5]

[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
> - [x] code pipeline testing (TestPeriodicGC fails without the unrelated fix in https://github.com/openjdk/shenandoah/pull/372)
> - [x] specjbb with SOEP=default,0,100 to verify that the issue is fixed

Y. Srinivas Ramakrishna has updated the pull request incrementally with one additional commit since the last revision:

  Changes stemming from Kelvin's review.

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/369/files
  - new: https://git.openjdk.org/shenandoah/pull/369/files/a290be10..87d45d5a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=369&range=04
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=369&range=03-04

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 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