RFR: 8369051: More small Float16 refactorings

[Joe Darcy]

As a follow-up to https://git.openjdk.org/jdk/pull/27587, I went through the code of Float16.java and found a few more cases where hex integer constants could be replaced by symbolic constants. A few other small changes included too.

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Commit messages:
 - JDK-8369051: More small Float16 refactorings

Changes: https://git.openjdk.org/jdk/pull/27610/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27610&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369051
  Stats: 6 lines in 1 file changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/27610.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27610/head:pull/27610

PR: https://git.openjdk.org/jdk/pull/27610