RFR: [type-classes] 8377322: Upgrade Float16 to use operators

[Joe Darcy]

Add witnesses and associated tests to Float16.

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Commit messages:
 - 8377322: Upgrade Float16 to use operators

Changes: https://git.openjdk.org/valhalla/pull/2051/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2051&range=00
  Stats: 190 lines in 3 files changed: 185 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/valhalla/pull/2051.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2051/head:pull/2051

PR: https://git.openjdk.org/valhalla/pull/2051