Simplify vacuum visibility assertion

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