The `popcnt` feature checks weren't actually checking the ISA feature
or cpuid. This caused an illegal instruction exception on x86 hardware
that doesn't support `popcnt` when compiled with MSVC.
Since MSVC doesn't support any x86 ISA defines, the only way to check
for `popcnt` is with cpuid. Since this is a tiny inline function,
dynamic dispatch is likley not worth it so just removing the MSVC
for `popcnt`. MSVC will now fallback to the generic implementation.
