Apply Tanner's suggested changes

Author: marek-safar

src/libraries/Common/src/System/HexConverter.cs

@@ -107,14 +107,28 @@ public static unsafe string ToString(ReadOnlySpan<byte> bytes, Casing casing = C
         public static char ToCharUpper(int value)
         {
             value &= 0xF;
-            return (char)(value > 9 ? value - 10 + 'A' : value + '0');
+            value += '0';
+
+            if (value > '9')
+            {
+                value += ('A' - '0');
+            }
+
+            return (char)value;
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static char ToCharLower(int value)
         {
             value &= 0xF;
-            return (char)(value > 9 ? value - 10 + 'a' : value + '0');
+            value += '0';
+
+            if (value > '9')
+            {
+                value += ('a' - '0');
+            }
+
+            return (char)value;
         }
     }
 }