Avoid looping through line pointers twice in PageRepairFragmentation().

Avoid looping through line pointers twice in PageRepairFragmentation().

There doesn't seem to be any good reason to do the filling of the
itemidbase[] array separately from the first traversal of the pointers.
It's certainly not a win if there are any line pointers with storage,
and even if there aren't, this change doesn't insert code into the part
of the first loop that will be traversed in that case.  So let's just
merge the two loops.

Yura Sokolov, reviewed by Claudio Freire

Discussion: https://postgr.es/m/e49befcc6f1d7099834c6fdf5c675a60@postgrespro.ru