RFR: 8365206: RISC-V: compiler/c2/irTests/TestFloat16ScalarOperations.java is failing on riscv64

[Hamlin Li]

Hi,
Can you help to review this patch?

Currently implementation of conversion from float to float16 only preserve some significant bits of a NaN, which is not right in some cases.
As this (NaN conversion from float to float16) is already in the slow path, so I'll just fix it by preserving all significant bits in the same way as j.l.Float.floatToFloat16.
As current tests does not catch the issue, except of TestFloat16ScalarOperations.java, so add another test.

There is also the similar issue in vector version of conversion from float to float16, I'll address it in another pr [JDK-8365772](https://bugs.openjdk.org/browse/JDK-8365772)

Thanks!

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/26838/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26838&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365206
  Stats: 230 lines in 6 files changed: 203 ins; 21 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/26838.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26838/head:pull/26838

PR: https://git.openjdk.org/jdk/pull/26838