[lworld+fp16] RFR: 8341368: Refactor to use nextUp/nextDown and scalb in Float16 impl. and tests

[Raffaello Giulietti]

Make use of `Float16.nextUp()`, `Float16.nextDown()`, `Float16.scalb()` in tests and code.

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Commit messages:
 - 8341368: Refactor to use nextUp/nextDown and scalb in Float16 impl. and tests

Changes: https://git.openjdk.org/valhalla/pull/1265/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1265&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341368
  Stats: 27 lines in 2 files changed: 0 ins; 16 del; 11 mod
  Patch: https://git.openjdk.org/valhalla/pull/1265.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1265/head:pull/1265

PR: https://git.openjdk.org/valhalla/pull/1265