[llvm] [VP][RISCV] Introduce experimental.vp.popcount and RISC-V support. (PR #74294)

[Yeting Kuo via llvm-commits]

yetingk wrote:

> Can this be expressed as a @llvm.vp.reduce.add with a start value of 0?

Of course. We could use `vp.zext` to transform mask vector to i32 vector and count the i32 vector by `vp.reduce.add`. But adding `vp.popcount` can reduce the complexity for vectorizer and lowering development.

https://github.com/llvm/llvm-project/pull/74294