[lworld+fp16] RFR: 8334432: Refine Float16.fma

[Joe Darcy]

Adding comments and test cases for Float16.fma.

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Commit messages:
 - JDK-8334432: Refine Float16.fma

Changes: https://git.openjdk.org/valhalla/pull/1143/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1143&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334432
  Stats: 189 lines in 2 files changed: 157 ins; 0 del; 32 mod
  Patch: https://git.openjdk.org/valhalla/pull/1143.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1143/head:pull/1143

PR: https://git.openjdk.org/valhalla/pull/1143