C++: Avoid creating ExprNodes for Conversions

Author: dave-bartolomeo

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -25,7 +25,12 @@ class Node extends Instruction {
   }
 
   /** Gets the expression corresponding to this node, if any. */
-  Expr asExpr() { result = this.getConvertedResultExpression() }
+  Expr asExpr() { 
+    result = this.getConvertedResultExpression() and
+    // Ignore conversions. The AST-based library does have an `ExprNode` for each `Conversion`, but
+    // there is no flow involving those nodes.
+    not result instanceof Conversion
+  }
 
   /** Gets the parameter corresponding to this node, if any. */
   Parameter asParameter() { result = this.(InitializeParameterInstruction).getParameter() }
@@ -100,7 +105,9 @@ abstract class PostUpdateNode extends Node {
 /**
  * Gets the `Node` corresponding to `e`.
  */
-ExprNode exprNode(Expr e) { result.getExpr() = e }
+ExprNode exprNode(Expr e) {
+  result.getExpr() = e
+}
 
 /**
  * Gets the `Node` corresponding to the value of `p` at function entry.