RFR: 8368985: Small Float16 refactorings

[Joe Darcy]

A few more small cleanups to Float16 and its tests.

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Commit messages:
 - JDK-8368985: Small Float16 refactorings

Changes: https://git.openjdk.org/jdk/pull/27587/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27587&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368985
  Stats: 20 lines in 2 files changed: 4 ins; 2 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/27587.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27587/head:pull/27587

PR: https://git.openjdk.org/jdk/pull/27587