RFR: 8295276: AArch64: Add backend support for half float conversion intrinsics

[Bhavana Kilambi]

This patch adds aarch64 backend support for library intrinsics that implement conversions between half-precision and single-precision floats.

Ran the following benchmarks to assess the performance with this patch -

org.openjdk.bench.java.math.Fp16ConversionBenchmark.floatToFloat16 org.openjdk.bench.java.math.Fp16ConversionBenchmark.float16ToFloat

The performance (ops/ms) gain with the patch on an ARM NEON machine is shown below -


  Benchmark                                      Gain
  Fp16ConversionBenchmark.float16ToFloat         3.42
  Fp16ConversionBenchmark.floatToFloat16         5.85

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Commit messages:
 - 8295276: AArch64: Add backend support for half float conversion intrinsics

Changes: https://git.openjdk.org/jdk/pull/10796/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10796&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295276
  Stats: 658 lines in 4 files changed: 34 ins; 0 del; 624 mod
  Patch: https://git.openjdk.org/jdk/pull/10796.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10796/head:pull/10796

PR: https://git.openjdk.org/jdk/pull/10796