RFR: 8339196: Optimize BufWriterImpl#writeU1/U2/Int/Long

[Shaojin Wen]

A small optimization makes BufWriterImpl's writeU1/U2/Int/Long methods more C2-friendly and improves performance.

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Commit messages:
 - refactor
 - optimize BufWriterImpl#writeU1/U2/Int/Long

Changes: https://git.openjdk.org/jdk/pull/20748/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20748&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339196
  Stats: 42 lines in 1 file changed: 32 ins; 3 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/20748.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20748/head:pull/20748

PR: https://git.openjdk.org/jdk/pull/20748