RFR: 8377355: VectorAPI source generation broken after JDK-8371187

[Varada M]

Added the missing definition of 'swapIfNeeded' in X-Vector.java.template 

JBS : [JDK-8377355](https://bugs.openjdk.org/browse/JDK-8377355)

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Commit messages:
 - JDK-8377355: VectorAPI source generation broken after JDK-8371187
 - JDK-8377355: VectorAPI source generation broken after JDK-8371187

Changes: https://git.openjdk.org/jdk/pull/29616/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29616&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377355
  Stats: 17 lines in 2 files changed: 16 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29616.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29616/head:pull/29616

PR: https://git.openjdk.org/jdk/pull/29616