RFR: 8329223: Parallel: Parallel GC resizes heap even has -Xms = -Xmx [v3]

[Zhengyu Gu]

> Parallel GC resizes old gen even when setting -Xms == -Xmx. 
> 
> During GC argument setup, it always set `MinOldSize = GenAlignment` and does not make any adjustment when `-Xms == -Xmx`, while does the adjustment for young gen [here](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shared/genArguments.cpp#L250)

Zhengyu Gu 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 four additional commits since the last revision:

 - albertnetymk's suggestion
 - Merge branch 'master' into JDK-8329223
 - Cleanup
 - 8329223: Parallel: Parallel GC resizes heap even has -Xms = -Xmx

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18539/files
  - new: https://git.openjdk.org/jdk/pull/18539/files/00c9d8b5..b6992f2b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18539&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18539&range=01-02

  Stats: 38617 lines in 974 files changed: 16290 ins; 14904 del; 7423 mod
  Patch: https://git.openjdk.org/jdk/pull/18539.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18539/head:pull/18539

PR: https://git.openjdk.org/jdk/pull/18539