UefiCpuPkg/MtrrLib: use new IS_POW2() macro.

Drop MtrrLibIsPowerOfTwo function, use the new IS_POW2() macro instead.

The ASSERT() removed (inside MtrrLibIsPowerOfTwo) is superfluous,
another ASSERT() a few lines up in MtrrLibCalculateMtrrs() already
guarantees that Length can not be zero at this point.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>