Use IndexTupleSize everywhere, instead.  Also, remove IndexTupleSize's
internal typecast, as that's not really needed and might mask coding
errors.  Change some pointer variable datatypes in the call sites
to compensate for that and make it clearer what we're assuming.

Ildar Musin, Robert Haas, Stephen Frost

Discussion: https://postgr.es/m/0274288e-9e88-13b6-c61c-7b36928bf221@postgrespro.ru