MdePkg/BaseMemoryLibOptDxe ARM: fix arithmetic bugs in CompareMem()

Fix two bugs:
- Erroneous shift of 2 in a bytes to bits conversion.
- Use reverse subtract rather than negate for value that is subsequently
  used as operand #2 in a shift operation.

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

MdePkg/Library/BaseMemoryLibOptDxe/Arm/CompareMem.S

@@ -110,9 +110,9 @@ ASM_PFX(InternalMemCompareMem):
     bic     src1, src1, #3
     bic     src2, src2, #3
     add     limit, limit, tmp1          // Adjust the limit for the extra.
-    lsl     tmp1, tmp1, #2              // Bytes beyond alignment -> bits.
+    lsl     tmp1, tmp1, #3              // Bytes beyond alignment -> bits.
     ldr     data1, [src1], #4
-    neg     tmp1, tmp1                  // Bits to alignment -32.
+    rsb     tmp1, tmp1, #32             // Bits to alignment -32.
     ldr     data2, [src2], #4
     mov     tmp2, #~0