RFR: 8295302: Do not use ArrayList when LambdaForm has a single ClassData

[Ioi Lam]

Please review this small optimization. As shown in the JBS issue, most of the generated LambdaForm classes have a single ClassData, so we can get a small footprint/speed improvement.

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Commit messages:
 - 8295302: Do not use ArrayList when LambdaForm has a single ClassData

Changes: https://git.openjdk.org/jdk/pull/10706/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10706&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295302
  Stats: 32 lines in 1 file changed: 20 ins; 8 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/10706.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10706/head:pull/10706

PR: https://git.openjdk.org/jdk/pull/10706