MdePkg/BaseMemoryLib: Fix VS2015 build error

MdePkg/BaseMemoryLib: Fix VS2015 build error

https://bugzilla.tianocore.org/show_bug.cgi?id=237

Make the smallest change possible to workaround a VS2015
build error.  The change is to the loop that handles the
case where neither the source nor the destination are
64-bit or 32-bit aligned and the logic falls through to
a loop that performs the copy as bytes.  Only the loop
that copies bytes backwards needs to be updated to avoid
the VS2015 build error.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>