RFR: 8318421 : AbstractPipeline.sourceStageSpliterator() chases pointers needlessly

[Viktor Klang]

Removes a few unnecessary dereferences of `sourceStage`.

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Commit messages:
 - Avoiding unnecessary pointer-chasing in sourceStageSpliterator() and avoiding a check-then-act in close()

Changes: https://git.openjdk.org/jdk/pull/16241/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16241&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318421
  Stats: 10 lines in 1 file changed: 1 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/16241.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16241/head:pull/16241

PR: https://git.openjdk.org/jdk/pull/16241