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

[Philip Reames via llvm-commits]

preames wrote:

Please rework your commit message.  If your goal is to exercise the costs, then you really should be writing a cost model test, not an SLP test.  I suspect that despite your wording, your actual intention is to add coverage showing that SLP can vectorize these examples on RISCV.  (The difference in wording there is that the cost of the ctpop expansion is only part of that.)

Note that we do appear to have cost model tests already in test/Analysis/CostModel/RISCV/int-bit-manip.ll.

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