Use bitwise test instead of popcount to check if one bit is set

Use bitwise test instead of popcount to check if one bit is set

We don't need to determine the total number of bits set to determine if
exactly one is set.

Additionally, on x86_64 without any -march=* flag, __builtin_popcount
will get compiled to a function call to the compiler runtime (on gcc),
or a long sequence of bit operations (on clang).

Signed-off-by: Michael Forney <mforney@mforney.org>