MdePkg/MemoryLib: Refine InternalMemSetMem16|32|64 functions logic

MdePkg/MemoryLib: Refine InternalMemSetMem16|32|64 functions logic

This commit refines the logic for InternalMemSetMem16|32|64 functions. It
avoids using the decrement operator '--' for array index to prevent
possible mis-reports by static code checkers.

Please note that those modified functions are only consumed within
MemoryLib by APIs SetMem16|32|64, and those APIs will handle the case when
the input number of bytes to set is 0. Hence, the behavior of APIs
SetMem16|32|64 is not changed.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>