RFR: 8321003: RISC-V: C2 MulReductionVI

[Hamlin Li]

Hi,
Can you help to review this patch to implement MulReductionVI/MulReductionVL/MulReductionVF/MulReductionVD?
On riscv, there is no straightforward instructions to do it, but we can do it with a reduction tree, which could reduce the time complexity to lg(N).
Thanks

## Performance
TBD

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/19015/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19015&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321003
  Stats: 150 lines in 9 files changed: 149 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19015.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19015/head:pull/19015

PR: https://git.openjdk.org/jdk/pull/19015