Simplify vacuum visibility assertion

Phase I vacuum gives the page a once-over after pruning and freezing to
check that the values of all_visible and all_frozen agree with the
result of heap_page_is_all_visible(). This is meant to keep the logic in
phase I for determining visibility in sync with the logic in phase III.

Rewrite the assertion to avoid an Assert(false).

Suggested by Andres Freund.

Author: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/mhf4vkmh3j57zx7vuxp4jagtdzwhu3573pgfpmnjwqa6i6yj5y%40sy4ymcdtdklo

src/backend/access/heap/vacuumlazy.c

@@ -2042,10 +2042,9 @@ lazy_scan_prune(LVRelState *vacrel,
 
 		Assert(presult.lpdead_items == 0);
 
-		if (!heap_page_is_all_visible(vacrel->rel, buf,
+		Assert(heap_page_is_all_visible(vacrel->rel, buf,
-									  vacrel->cutoffs.OldestXmin, &debug_all_frozen,
+										vacrel->cutoffs.OldestXmin, &debug_all_frozen,
-									  &debug_cutoff, &vacrel->offnum))
+										&debug_cutoff, &vacrel->offnum));
-			Assert(false);
 
 		Assert(presult.all_frozen == debug_all_frozen);