MdePkg/BaseMemoryLibOptDxe ARM: fix Thumb-2 bug in ScanMem()

MdePkg/BaseMemoryLibOptDxe ARM: fix Thumb-2 bug in ScanMem()

The ARM ScanMem() in BaseMemoryLibOptDxe contains code from the open
source cortex-strings library, and inherited a bug from it where the
conditional execution of a sequence of instructions is erroneously
made dependent on the same condition. Since the final 'addeq' is
supposed to be dependent on the preceding 'tsteq' instruction, they
cannot be part of the same IT block.

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