RFR: 8321939: [GenShen] ShenandoahOldEvacRatioPercent=100 fails with divide-by-zero [v2]

[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:**
> - [ ] GHA
> - [ ] code pipeline testing
> - [ ] 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:

  Fix a scope issue with a local variable.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/369/files
  - new: https://git.openjdk.org/shenandoah/pull/369/files/7166147f..03e55dee

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=369&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=369&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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