C#: Fix bad join in cs/coalesce-of-identical-expressions.

Author: michaelnebel

csharp/ql/src/Language Abuse/UselessNullCoalescingExpression.ql

@@ -15,11 +15,16 @@
 import csharp
 import semmle.code.csharp.commons.StructuralComparison
 
+pragma[nomagic]
+private Expr getARightChildExpr(NullCoalescingOperation nce) {
+  result = nce.getRightOperand().getAChildExpr*()
+}
+
 pragma[noinline]
 private predicate same(AssignableAccess x, AssignableAccess y) {
   exists(NullCoalescingOperation nc |
     x = nc.getLeftOperand() and
-    y = nc.getRightOperand().getAChildExpr*()
+    y = getARightChildExpr(nc)
   ) and
   sameGvn(x, y)
 }