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

Recently, 7f26c27e turned on SLP by default for RISC-V, and although
there are quite a few tests for SLP under the X86/ target, it is unclear
whether the same constructs would be vectorized on RISC-V. This patch
takes a step in the direction of remedying this, by noticing that ctpop
is often vectorized on RISC-V, and adding four tests for different
integer widths.