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

[Philip Reames via llvm-commits]

preames wrote:

LGTM as well.

For your follow up on this, note that vector ctpop exists only under ZVBB.  I suspect we didn't update the costs for ctpop if zvbb is enabled.  Without it, vectorizing is probably not profitable with the default expansion.  That'd be the first thing I'd check.  If my guess is right, you're also want a CostModel test.  

@lukel97 FYI  

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