C#: Reduce reliance on CFG nodes in DataFlow::Node.

Author: aschackmull

csharp/ql/consistency-queries/DataFlowConsistency.ql

@@ -71,7 +71,7 @@ private module Input implements InputSig<Location, CsharpDataFlow> {
         )
       or
       exists(ControlFlow::Nodes::ElementNode cfn, ControlFlow::Nodes::Split split |
-        exists(arg.asExprAtNode(cfn))
+        arg.asExpr().getControlFlowNode() = cfn
       |
         split = cfn.getASplit() and
         not split = call.getControlFlowNode().getASplit()

csharp/ql/lib/semmle/code/csharp/controlflow/Guards.qll

@@ -784,11 +784,7 @@ class GuardedDataFlowNode extends DataFlow::ExprNode {
   private AccessOrCallExpr sub0;
   private GuardValue v0;
 
-  GuardedDataFlowNode() {
-    exists(ControlFlow::Nodes::ElementNode cfn | exists(this.getExprAtNode(cfn)) |
-      g.controlsNode(cfn, sub0, v0)
-    )
-  }
+  GuardedDataFlowNode() { g.controlsNode(this.getExpr().getControlFlowNode(), sub0, v0) }
 
   /**
    * Gets an expression that guards this data flow node. That is, this data flow