Fix pg_visibility regression failure with CLOBBER_CACHE_ALWAYS

Commit 8e03eb92 reverted a bit too much code, reintroducing one of the
issues fixed by 39b66a91 - a page might have been left partially empty
after relcache invalidation.

Reported-By: Tom Lane
Author: Masahiko Sawada
Discussion: https://postgr.es/m/822752.1623032114@sss.pgh.pa.us
Discussion: https://postgr.es/m/CAD21AoA%3D%3Df2VSw3c-Cp_y%3DWLKHMKc1D6s7g3YWsCOvgaYPpJcg%40mail.gmail.com

src/backend/access/heap/hio.c

@@ -407,6 +407,7 @@ RelationGetBufferForTuple(Relation relation, Size len,
 		 * target.
 		 */
 		targetBlock = GetPageWithFreeSpace(relation, targetFreeSpace);
+	}
 
 	/*
 	 * If the FSM knows nothing of the rel, try the last page before we
@@ -420,7 +421,6 @@ RelationGetBufferForTuple(Relation relation, Size len,
 		if (nblocks > 0)
 			targetBlock = nblocks - 1;
 	}
-	}
 
 loop:
 	while (targetBlock != InvalidBlockNumber)