C++: Fix getTempVariable join order in IR

This join order seems to have broken so it took forever on
wireshark/wireshark.

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/internal/TranslatedElement.qll

@@ -684,9 +684,17 @@ abstract class TranslatedElement extends TTranslatedElement {
    * Gets the temporary variable generated by this element with tag `tag`.
    */
   final IRTempVariable getTempVariable(TempVariableTag tag) {
-    result.getAST() = getAST() and
-    result.getTag() = tag and
-    hasTempVariable(tag, _)
+    exists(Locatable ast |
+      result.getAST() = ast and
+      result.getTag() = tag and
+      hasTempVariableAndAST(tag, ast)
+    )
+  }
+
+  pragma[noinline]
+  private predicate hasTempVariableAndAST(TempVariableTag tag, Locatable ast) {
+    hasTempVariable(tag, _) and
+    ast = getAST()
   }
 
   /**