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

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

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

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