[jdk21u-dev] RFR: 8329223: Parallel: Parallel GC resizes heap even if -Xms = -Xmx

[Ben Taylor]

Clean backport of a low-risk change to address unexpected behavior with Parallel GC when Xms == Xmx
Passes local tier-1 testing on linux x86_64

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Commit messages:
 - Backport aca1e8365bf0f64bf18caf798bbca1d25b3c4117

Changes: https://git.openjdk.org/jdk21u-dev/pull/587/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=587&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329223
  Stats: 11 lines in 1 file changed: 6 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/587.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/587/head:pull/587

PR: https://git.openjdk.org/jdk21u-dev/pull/587