New compiler warning from btree dedup code

[Tom Lane]

I just noticed that if I build without asserts on my Mac laptop
(using Apple's latest clang, 13.1.6) I get

nbtdedup.c:68:8: warning: variable 'pagesaving' set but not used 
[-Wunused-but-set-variable]
        Size            pagesaving = 0;
                        ^
1 warning generated.

Apparently, late-model clang can figure out that the variable
is incremented but not otherwise used.  This is enough to
shut it up, but I wonder if you have another preference:

-   Size        pagesaving = 0;
+   Size        pagesaving PG_USED_FOR_ASSERTS_ONLY = 0;

                        regards, tom lane