Use POPCNT on MSVC

Going by [1], it looks like we can use the __popcnt and __popcnt64
intrinsic functions on MSVC if the CPU supports POPCNT.  We already
have code to check for that, we just need to enable it on MSVC.

The attached patch seems to be all that's needed.

David

[1] https://docs.microsoft.com/en-us/cpp/intrinsics/popcnt16-popcnt-popcnt64?view=msvc-140