[llvm] SLP/RISCV: add test for vectorized ctpop, like in X86 (PR #65330)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> Why specifically `ctpop`.

I was investigating some benchmarks where I came across `ctpop` being vectorized by gcc-aarch64, but not llvm-riscv, so I was curious about whether LLVM does any vectorization of `ctpop` at all on RISC-V. This patch is a result of that investigation; nothing more.

> There are much more coverage for the X86 backend, is the whole plan to add them commit by commit to the RISC-V backend?

Do you have a different suggestion?

https://github.com/llvm/llvm-project/pull/65330