RFR: 8358529: GenShen: Heuristics do not respond to changes in SoftMaxHeapSize [v2]

[Rui Li]

> Generational shenandoah currently doesn't pick up the changes of managed flag `SoftMaxHeapSize` when the app is running. This is because the value of `_soft_max_capacity` in `shenandoahGeneration` is never changed.
> 
> This change delegates the soft max heap size in `shenandoahGeneration` to `ShenandoahGenerationalHeap::heap()->soft_max_capacity()`, which does pick up the flag value changes.
> 
> Also, found `ShenandoahHeap:: initialize` uses `_num_regions * reg_size_bytes` rather than user input flag value. Updated to using actual flag value.

Rui Li has updated the pull request incrementally with one additional commit since the last revision:

  No need atomic load in initialization

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25943/files
  - new: https://git.openjdk.org/jdk/pull/25943/files/d5dda920..49bda04e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25943&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25943&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25943.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25943/head:pull/25943

PR: https://git.openjdk.org/jdk/pull/25943