ArmPkg: Fix Ecc error 3002 in CompilerIntrinsicsLib

This patch fixes the following Ecc reported error:
Non-Boolean comparisons should use a compare operator
(==, !=, >, < >=, <=)

Brackets are also added to comply to with the coding
standard.

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>

ArmPkg/Library/CompilerIntrinsicsLib/memcmp_ms.c

@@ -1,6 +1,7 @@
 //------------------------------------------------------------------------------
 //
 // Copyright (c) 2019, Pete Batard. All rights reserved.
+// Copyright (c) 2021, Arm Limited. All rights reserved.<BR>
 //
 // SPDX-License-Identifier: BSD-2-Clause-Patent
 //
@@ -20,7 +21,7 @@ int memcmp(const void *s1, const void *s2, size_t n)
   unsigned char const *t1 = s1;
   unsigned char const *t2 = s2;
 
-  while (n--) {
+  while (n-- != 0) {
     if (*t1 != *t2)
       return (int)*t1 - (int)*t2;
     t1++;