RFR: 8339158: TypeKind add slotSize field

[Shaojin Wen]

Small improvement, add a final field, no need to calculate every time.

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Commit messages:
 - slotSize field

Changes: https://git.openjdk.org/jdk/pull/20743/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20743&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339158
  Stats: 18 lines in 1 file changed: 2 ins; 4 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/20743.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20743/head:pull/20743

PR: https://git.openjdk.org/jdk/pull/20743