heap_prepare_freeze_tuple: Simplify coding

Commit d2599ecf introduced some contorted, confused code around:
readers would think that it's possible for HeapTupleHeaderGetXmin return
a non-frozen value for some frozen tuples, which would be disastrous.
There's no actual bug, but it seems better to make it clearer.

Per gripe from Tom Lane and Andres Freund.
Discussion: https://postgr.es/m/30116.1555430496@sss.pgh.pa.us