Clean up the usage of canonicalize_qual(): in particular, be consistent

Clean up the usage of canonicalize_qual(): in particular, be consistent
about whether it is applied before or after eval_const_expressions().
I believe there were some corner cases where the system would fail to
recognize that a partial index is applicable because of the previous
inconsistency.  Store normal rather than 'implicit AND' representations
of constraints and index predicates in the catalogs.
initdb forced due to representation change of constraints/predicates.