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

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

Using a zext + add reduction pattern looks to be a better approach tbh - and even if we needed this intrinsic, we'd want to rename it as vp.popcount suggests a per-element instrinsic.

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