C#: Prevent bad magic in a few predicates.

csharp/ql/lib/semmle/code/csharp/commons/ComparisonTest.qll

@@ -305,6 +305,7 @@ class ComparisonTest extends TComparisonTest {
   }
 
   /** Gets an argument of this comparison test. */
+  pragma[nomagic]
   Expr getAnArgument() {
     result = this.getFirstArgument() or
     result = this.getSecondArgument()

csharp/ql/lib/semmle/code/csharp/exprs/Call.qll

@@ -40,6 +40,7 @@ class Call extends Expr, @call {
   Callable getTarget() { none() }
 
   /** Gets the `i`th argument to this call, if any. */
+  pragma[nomagic]
   Expr getArgument(int i) { result = this.getChild(i) and i >= 0 }
 
   /**