Do not convert an expression node that is already TK_REGISTER into a

new TK_REGISTER.  This is a follow-up to [663f5dd32d9db832] that fixes a
bug discovered by dbsqlfuzz.

FossilOrigin-Name: fcb669cfaa8a59ca710504d5aaa1936f65a6da8ff13473ad84458f97a6fd1f49

test/distinct2.test

@@ -362,4 +362,22 @@ do_execsql_test 5070 {
   SELECT v4.e FROM t3 LEFT JOIN v4 ON true GROUP BY 1;
 } NULL
 
+# 2024-06-28 dbsqlfuzz 46343912848a603e32c6072cae792eb056bac897
+# Do not call sqlite3ExprToRegister() on an expression that is already
+# a register.
+#
+do_execsql_test 5080 {
+  CREATE TABLE dual(dummy TEXT);
+  INSERT INTO dual VALUES('X');
+  SELECT 11 = (
+                SELECT b
+                  FROM (
+                         SELECT a AS b
+                           FROM dual
+                           LEFT JOIN (SELECT 22 AS a FROM dual)
+                       )
+                 GROUP BY b, b
+              );
+} 0
+
 finish_test