Missing support for llvm.ctpop.* intrinsics

[connorskees]

https://llvm.org/docs/LangRef.html#llvm-ctpop-intrinsic

There are a number of ways to implement this, but I haven't looked into which solution would work best with boolector. My assumption is https://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel would be fastest.