[vectorIntrinsics+fp16] RFR: Conversion routines for FP16 from jdk mainline

[Swati Sharma]

Hi All,

The patch contains conversion routines for FP16 from jdk mainline.
-	This is an interim solution till a full blown merge is done from mainline to vectorIntrinsics and from vectorIntrinsics to vectorIntrinsics+fp16.

Please review and provide your feedback.

Swati

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Commit messages:
 - Merge jdk-mainline fp16 changes.

Changes: https://git.openjdk.org/panama-vector/pull/206/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-vector&pr=206&range=00
  Stats: 2362 lines in 5 files changed: 2284 ins; 46 del; 32 mod
  Patch: https://git.openjdk.org/panama-vector/pull/206.diff
  Fetch: git fetch https://git.openjdk.org/panama-vector pull/206/head:pull/206

PR: https://git.openjdk.org/panama-vector/pull/206