RFR: 8329223: Parallel: Parallel GC resizes heap even has -Xms = -Xmx [v6]

[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 incrementally with one additional commit since the last revision:

  Albert's comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18539/files
  - new: https://git.openjdk.org/jdk/pull/18539/files/2ce48272..e869ee8e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18539&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18539&range=04-05

  Stats: 6 lines in 1 file changed: 3 ins; 3 del; 0 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