[vectorIntrinsics+fp16] RFR: 8299373: Extend vector conversion APIs for Halffloat type.

[Swati Sharma]

Hi All,

The patch contains the below changes:

- Extended existing conversion APIs (convertShape, castShape, convert) for half float vectors.
- Define new conversion operators for casting to/from halffloat type.
- Extended existing JTREG conversion tests for half float types.

Please review and provide your feedback.

Thanks,
Swati

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Commit messages:
 - 8299373: Extend vector conversion APIs for Halffloat type.

Changes: https://git.openjdk.org/panama-vector/pull/208/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-vector&pr=208&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299373
  Stats: 123 lines in 5 files changed: 106 ins; 1 del; 16 mod
  Patch: https://git.openjdk.org/panama-vector/pull/208.diff
  Fetch: git fetch https://git.openjdk.org/panama-vector pull/208/head:pull/208

PR: https://git.openjdk.org/panama-vector/pull/208