Check Intel compiler before MSVC in bitcount.h

Predefined macro __INTEL_COMPILER is defined only for Intel,
while _MSC_VER is defined for both Intel C++ and MSVC.

So rearrange ifdefs to take in account this and test __INTEL_COMPILER
first and only if not defined check _MSC_VER for MSVC.

Patch suggested by Joona.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>