From how I read this comment, it's not exposed on x86 because it's extremely slow on Atom:

https://github.com/dotnet/coreclr/blob/cb9cbef5cd036bb761720...

When I needed these instructions, I created this https://github.com/omgtehlion/netintrinsics

Guys in OP seem to be more clever, though

bitscan would change 3 instructions (xor, mul, -1) with one - wouldn't be a huge difference; in this particular scenario.