[nikic]

The background here is that "ctpop < 2" or "ctpop == 1" (depending on zero behavior) is LLVM's canonical representation for a "power of two" check. It is used on the premise that the backend will expand it back into a cheap bitwise check and not use an actual ctpop operation. However, due to complex interactions in the backend, this does not actually happen in this case (https://github.com/llvm/llvm-project/issues/94829).