RFR: 8341136: Optimize StackMapGenerator::trimAndCompress

[Shaojin Wen]

A small optimization to reduce the write operations of trimAndCompress

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Commit messages:
 - optimize trimAndCompress

Changes: https://git.openjdk.org/jdk/pull/21227/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21227&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341136
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21227.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21227/head:pull/21227

PR: https://git.openjdk.org/jdk/pull/21227