changed for VC compiler and 32bit platform

For VC compiler and 32bit platform, function libdivide__count_leading_zeros64 changed to use _BitScanReverse intinsic function instead of while loop. Tested only on Windows CE 6.0 ARM Cortex A8 and libdivide::divider<unsigned __int64l> perfoms about 10-20% faster than the original implementation. The _BitScanReverse is not implemented under CE VC2008 and was implemented by me in separate file as 31-_CountLeadingZeros.
You are right, this mask could be eliminated, but instead casting to uint32_t should be added. Without the mask or type casting, VC generates warning "conversion from 'uint64_t' to 'uint32_t', possible loss of data".
I'm newbe to git and github, should I now correct this in my fork and then pull new request  ?