RFR: 8369123: Still more small Float16 refactorings

[Joe Darcy]

Upon further inspection, found a few more opportunities to refine the use of constants in the Float16 implementation.

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Commit messages:
 - JDK-8369123: Still more small Float16 refactorings

Changes: https://git.openjdk.org/jdk/pull/27625/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27625&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369123
  Stats: 11 lines in 1 file changed: 2 ins; 3 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/27625.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27625/head:pull/27625

PR: https://git.openjdk.org/jdk/pull/27625