[code-reflection] Integrated: Cleanup of FP16Phase removing duplicate code and leaning on InvokHelper

[Gary Frost]

Trying to simplify FP16Phase code (using InvokeHelper) and removing duplicate code patterns.

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Commit messages:
 - Some cleanup of FP16 phase removing duplicate code and leaning on InvokeHelper

Changes: https://git.openjdk.org/babylon/pull/801/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=801&range=00
  Stats: 1416 lines in 30 files changed: 411 ins; 452 del; 553 mod
  Patch: https://git.openjdk.org/babylon/pull/801.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/801/head:pull/801

PR: https://git.openjdk.org/babylon/pull/801