Fix IndexOutOfRangeException in CallSite.MoveRule. (#39159)

Because synchronization of AddRule is omitted for performance,
concurrent invocations of AddRule may cause Rules to revert back to
an older (smaller) version, making i out of bounds.
Add a guard against that.

The reasoning behind no synchronization is that Rules is just a cache
after all. As long as the cache is in a consistent state, wrong updates
will only cause more cache misses or longer lookup time.
This performance loss is fine if we get more performance gain by
omitting synchronization.

Fix #36983

Author: asmichi

src/libraries/System.Linq.Expressions/src/System/Runtime/CompilerServices/CallSite.cs

@@ -266,11 +266,16 @@ internal void MoveRule(int i)
             if (i > 1)
             {
                 T[] rules = Rules!;
-                T rule = rules[i];
+                // Synchronization of AddRule is omitted for performance. Concurrent invocations of AddRule
+                // may cause Rules to revert back to an older (smaller) version, making i out of bounds.
+                if (i < rules.Length)
+                {
+                    T rule = rules[i];
 
-                rules[i] = rules[i - 1];
-                rules[i - 1] = rules[i - 2];
-                rules[i - 2] = rule;
+                    rules[i] = rules[i - 1];
+                    rules[i - 1] = rules[i - 2];
+                    rules[i - 2] = rule;
+                }
             }
         }