RFR: 8339168: Optimize ClassFile Util slotSize

[Shaojin Wen]

A small optimization to improve the performance of jdk.internal.classfile.impl.Util#slotSize/isDoubleSlot

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Commit messages:
 - optimize slotSize & isDoubleSlot

Changes: https://git.openjdk.org/jdk/pull/20747/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20747&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339168
  Stats: 11 lines in 1 file changed: 4 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20747.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20747/head:pull/20747

PR: https://git.openjdk.org/jdk/pull/20747