MdePkg/BaseMemoryLibOptDxe: replace deprecated uses of IT blocks

The ARM architecture version 8 deprecates all uses of the IT instruction
except cases where it is followed by a single narrow instruction. So
replace any occurrences with equivalent sequences that adhere to the
new rules.

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/ScanMem.S

@@ -123,11 +123,12 @@ ASM_PFX(InternalMemScanMem8):
 60: // We're here because the fast path found a hit - now we have to track down exactly which word it was
     // r0 points to the start of the double word after the one that was tested
     // r5 has the 00/ff pattern for the first word, r6 has the chained value
+    subs    r0, r0, #3
     cmp     r5, #0
-    itte    eq
-    moveq   r5, r6        // the end is in the 2nd word
-    subeq   r0, r0, #3    // Points to 2nd byte of 2nd word
-    subne   r0, r0, #7    // or 2nd byte of 1st word
+    it      eq
+    moveq.n r5, r6        // the end is in the 2nd word
+    it      ne
+    subne.n r0, r0, #4    // or 2nd byte of 1st word
 
     // r0 currently points to the 3rd byte of the word containing the hit
     tst     r5, #CHARTSTMASK(0)     // 1st character