RFR: 8338780: GenShen: Fix up some comments

[Y. Srinivas Ramakrishna]

On Thu, 22 Aug 2024 17:20:49 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> In https://github.com/openjdk/jdk/pull/20395 Aleksey provided some suggestions for improving various comments. This ticket gathers together several of those changes.
> 
> **Testing:**
> - [x] GHA
> - [x] SPECjbb w/GenShen
> - [x] jtreg:tier1
> - [ ] Codepipeline testing

src/hotspot/share/gc/shenandoah/heuristics/shenandoahGlobalHeuristics.cpp line 115:

> 113:     ShenandoahHeapRegion* r = data[idx].get_region();
> 114:     if (cset->is_preselected(r->index())) {
> 115:       assert(false, "There should be no preselected regions during GLOBAL GC");

If this is a true invariant (which it is in testing to date), then it must be the case that the call at line 55 above to `add_preselected_regions_to_collection_set()` doesn't do anything, and indeed returns `0` into `cur_young_garbage` there.

Clearly, I am missing something here. @kdnilsen ?

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/484#discussion_r1727547920