C++: Use localExprFlow in existing queries

This shortens the queries a bit and ensures test coverage of the new
predicate.

cpp/ql/src/Critical/NewDelete.qll

@@ -47,7 +47,7 @@ predicate allocExprOrIndirect(Expr alloc, string kind) {
       or
       exists(Expr e |
         allocExprOrIndirect(e, kind) and
-        DataFlow::localFlow(DataFlow::exprNode(e), DataFlow::exprNode(rtn.getExpr()))
+        DataFlow::localExprFlow(e, rtn.getExpr())
       )
     )
   )

cpp/ql/src/Critical/OverflowStatic.ql

@@ -95,7 +95,7 @@ class CallWithBufferSize extends FunctionCall {
 
   int statedSizeValue() {
     exists(Expr statedSizeSrc |
-      DataFlow::localFlow(DataFlow::exprNode(statedSizeSrc), DataFlow::exprNode(statedSizeExpr())) and
+      DataFlow::localExprFlow(statedSizeSrc, statedSizeExpr()) and
       result = statedSizeSrc.getValue().toInt()
     )
   }