Fix style violations in syscache lookups.

Project style is to check the success of SearchSysCacheN and friends
by applying HeapTupleIsValid to the result.  A tiny minority of calls
creatively did it differently.  Bring them into line with the rest.

This is just cosmetic, since HeapTupleIsValid is indeed just a null
check at the moment ... but that may not be true forever, and in any
case it puts a mental burden on readers who may wonder why these
call sites are not like the rest.

Back-patch to v11 just to keep the branches in sync.  (The bulk of these
errors seem to have originated in v11 or v12, though a few are old.)

Per searching to see if anyplace else had made the same error
repaired in 62148c352.

src/backend/commands/indexcmds.c

@@ -1219,7 +1219,7 @@ DefineIndex(Oid relationId,
 
 				tup = SearchSysCache1(INDEXRELID,
 									  ObjectIdGetDatum(indexRelationId));
-				if (!tup)
+				if (!HeapTupleIsValid(tup))
 					elog(ERROR, "cache lookup failed for index %u",
 						 indexRelationId);
 				newtup = heap_copytuple(tup);