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

[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)

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Commit messages:
 - 8329223: Parallel: Parallel GC resizes heap even has -Xms = -Xmx

Changes: https://git.openjdk.org/jdk/pull/18539/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18539&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329223
  Stats: 6 lines in 2 files changed: 6 ins; 0 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