[code-reflection] Integrated: More FP16Phase cleanup. Leveraging Invoke helper

[Gary Frost]

Simplified FP16Phase code (transforms) using InvokeHelper and avoiding unnecessary multimapping.

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Commit messages:
 - More FP16Phase cleanup. Leveraging Invoke helper

Changes: https://git.openjdk.org/babylon/pull/803/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=803&range=00
  Stats: 261 lines in 3 files changed: 94 ins; 100 del; 67 mod
  Patch: https://git.openjdk.org/babylon/pull/803.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/803/head:pull/803

PR: https://git.openjdk.org/babylon/pull/803