[lworld+fp16] RFR: 8330021: AArch64: Add backend support for FP16 add operation [v3]

[Bhavana Kilambi]

> This commit [1] adds initial support for FP16 operations and adds backend support for FP16 add operation for X86. This task adds backend support for scalar and vector FP16 add operation on aarch64.
> 
> [1] https://github.com/openjdk/valhalla/commit/f03fb4e4ee4d59ed692d0c26ddce260511f544e7#diff-a799ce8da7f3062bb3699beb65aae504840c649942032e325c2a50f88a2869ad

Bhavana Kilambi has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Remove couple of IR testcases + add --enable-preview flag to tests
 - Merge branch 'lworld+fp16'
 - Add a missing return statement in the ad file
 - 8330021: AArch64: Add backend support for FP16 add operation
   
   This commit [1] adds initial support for FP16 operations and adds
   backend support for FP16 add operation for X86. This task adds backend
   support for scalar and vector FP16 add operation on aarch64.
   
   [1] https://github.com/openjdk/valhalla/commit/f03fb4e4ee4d59ed692d0c26ddce260511f544e7#diff-a799ce8da7f3062bb3699beb65aae504840c649942032e325c2a50f88a2869ad

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Changes: https://git.openjdk.org/valhalla/pull/1096/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1096&range=02
  Stats: 874 lines in 19 files changed: 176 ins; 6 del; 692 mod
  Patch: https://git.openjdk.org/valhalla/pull/1096.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1096/head:pull/1096

PR: https://git.openjdk.org/valhalla/pull/1096