Simplify GetOperatorFromCompareType() code

Simplify GetOperatorFromCompareType() code

The old code would set *opid = InvalidOid to determine if the
get_opclass_opfamily_and_input_type() worked or not.  This means more
moving parts that what's really needed here.  Let's just fail
immediately if the get_opclass_opfamily_and_input_type() lookup fails.

Author: Paul A Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Neil Chen <carpenter.nail.cz@gmail.com>
Discussion: https://postgr.es/m/CA+renyXOrjLacP_nhqEQUf2W+ZCoY2q5kpQCfG05vQVYzr8b9w@mail.gmail.com