Add assertion to verify derived clause has constant RHS

Add assertion to verify derived clause has constant RHS

find_derived_clause_for_ec_member() searches for a previously-derived
clause that equates a non-constant EquivalenceMember to a constant.
It is only called for EquivalenceClasses with ec_has_const set, and
with a non-constant member the EquivalenceMember to search for.

The matched clause is expected to have the non-constant member on the
left-hand side and the constant EquivalenceMember on the right.

Assert that the RHS is indeed a constant, to catch violations of this
structure and enforce assumptions made by
generate_base_implied_equalities_const().

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/CAExHW5scMxyFRqOFE6ODmBiW2rnVBEmeEcA-p4W_CyuEikURdA@mail.gmail.com