RFR: 8345146: [PPC64] Make intrinsic conversions between bit representations of half precision values and floats [v3]

[Martin Doerr]

> PPC64 implementation of [JDK-8289552](https://bugs.openjdk.org/browse/JDK-8289552). I've implemented some more instructions which may be useful in the future.
> VectorCastNodes are not yet implemented on PPC64. Power9 is recognized by the availability of the "darn" instruction.
> 
> Performance on Power9:
> Before patch:
> 
> Benchmark                                     (size)   Mode  Cnt      Score     Error   Units
> Fp16ConversionBenchmark.float16ToFloat          2048  thrpt   15     18.995 ?   0.156  ops/ms
> Fp16ConversionBenchmark.floatToFloat16          2048  thrpt   15     18.730 ?   0.331  ops/ms
> 
> 
> After patch:
> 
> Benchmark                                     (size)   Mode  Cnt      Score      Error   Units
> Fp16ConversionBenchmark.float16ToFloat          2048  thrpt   15    522.637 ?   11.274  ops/ms
> Fp16ConversionBenchmark.floatToFloat16          2048  thrpt   15    408.112 ?    9.069  ops/ms

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Make sure interpreter entries are not called on Power8 or older.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/22433/files
  - new: https://git.openjdk.org/jdk/pull/22433/files/02e54629..9a8a2734

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22433&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22433&range=01-02

  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/22433.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22433/head:pull/22433

PR: https://git.openjdk.org/jdk/pull/22433