RFR: 8340587: Optimize StackMapGenerator$Frame::checkAssignableTo

[Shaojin Wen]

Optimize checkAssignableTo to avoid clone when stackSize is 0, and use clone instead of Array.copyOf to avoid compression and then expansion

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Commit messages:
 - suggestion from @liach
 - optimize checkAssignableTo

Changes: https://git.openjdk.org/jdk/pull/21121/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21121&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340587
  Stats: 7 lines in 1 file changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21121.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21121/head:pull/21121

PR: https://git.openjdk.org/jdk/pull/21121