RFR: 8320995: RISC-V: C2 PopCountVI [v3]

[Hamlin Li]

> Hi,
> Can you help to review this patch?
> Both auto-vect and vector api depends on this intrinsic.
> Thanks!
> 
> ## Performance
> Not performance test was done, as this depends on vcpop.v instruction in zvbb extension and the code seqeunce is rather simple than non-intrinsic version.

Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:

  mark UseZvbb experimenal

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19065/files
  - new: https://git.openjdk.org/jdk/pull/19065/files/74692e23..f316f660

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19065&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19065&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19065.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19065/head:pull/19065

PR: https://git.openjdk.org/jdk/pull/19065