RFR: 8373998: RISC-V: simple optimization of ConvHF2F

[Hamlin Li]

Hi,
Can you help to review this patch?

ConvHF2F could be optimized by following patch.
As riscv does not have the restriction to use `iRegINoSp` in src register, it can use iRegIorL2I instead.
Check other usages of src register in riscv.ad file.

@RealFYang Thanks!

Thanks!

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/28896/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28896&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373998
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28896.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28896/head:pull/28896

PR: https://git.openjdk.org/jdk/pull/28896