Fix new-to-v19 -Wshadow warnings

Fix new-to-v19 -Wshadow warnings

There's some talk about upgrading our current -Wshadow=compatible-local
up to -Wshadow.  There's some pending questions as to whether the churn
and extra backpatching pain are worthwhile for doing all of them.  We
can't use the latter argument for ones that are new to v19, providing we
fix them now.  So let's fix those ones so that the problem is not any
worse for if we decide to fix the remainder for v20.

Author: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Yuchen Li <liyuchen_xyz@163.com>
Discussion: https://postgr.es/m/CAApHDvp=rx5GxM=yW8QhFF3noXtYt7LkOxJ7zkaPOzpti4Gm8w@mail.gmail.com