C#: Speed up ForwarderAssertMethod

Avoids the following bad predicate

```
[2023-08-29 10:03:13] (252s) Tuple counts for _Callable#f85cebf6::Callable::getBody#0#dispred#ff_Variable#afb43847::Variable::getAnAccess#0#dispre__#join_rhs/5@43feb6tl after 4m0s:
                      4416261    ~203%     {4} r1 = JOIN _Callable#f85cebf6::Callable::getAParameter#0#dispred#ff_10#join_rhs_Variable#afb43847::Variable::ge__#shared WITH Callable#f85cebf6::Callable::getBody#0#dispred#ff ON FIRST 1 OUTPUT Lhs.1 'arg1', Lhs.2 'arg2', Lhs.0 'arg3', Rhs.1 'arg4'
                      1189565718 ~152%     {5} r2 = JOIN r1 WITH Variable#afb43847::Variable::getAnAccess#0#dispred#ff ON FIRST 1 OUTPUT Rhs.1 'arg0', Lhs.0 'arg1', Lhs.1 'arg2', Lhs.2 'arg3', Lhs.3 'arg4'
                                           return r2
```

Author: hvitved

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

@@ -299,6 +299,12 @@ class NUnitAssertNonNullMethod extends NullnessAssertMethod, NUnitAssertMethod {
   override int getAnAssertionIndex(boolean b) { result = this.getAnAssertionIndex() and b = false }
 }
 
+pragma[nomagic]
+private predicate parameterAssertion(Assertion a, int index, Parameter p) {
+  strictcount(AssignableDefinition def | def.getTarget() = p) = 1 and
+  a.getExpr(index) = p.getAnAccess()
+}
+
 /** A method that forwards to another assertion method. */
 class ForwarderAssertMethod extends AssertMethod {
   private Assertion a;
@@ -307,10 +313,9 @@ class ForwarderAssertMethod extends AssertMethod {
 
   ForwarderAssertMethod() {
     p = this.getAParameter() and
-    strictcount(AssignableDefinition def | def.getTarget() = p) = 1 and
     forex(ControlFlowElement body | body = this.getBody() |
       bodyAsserts(this, body, a) and
-      a.getExpr(forwarderIndex) = p.getAnAccess()
+      parameterAssertion(a, forwarderIndex, p)
     )
   }