RFR: 8368822: Refactor Float16.valueOf(double)

[Joe Darcy]

Refactoring to express the code more in terms of symbolic concepts rather than raw constants.

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Commit messages:
 - JDK-8368822: Refactor Float16.valueOf(double)

Changes: https://git.openjdk.org/jdk/pull/27540/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27540&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368822
  Stats: 26 lines in 1 file changed: 8 ins; 4 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/27540.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27540/head:pull/27540

PR: https://git.openjdk.org/jdk/pull/27540