Teach the planner to treat a partial unique index as proving a variable is

Teach the planner to treat a partial unique index as proving a variable is
unique for a particular query, if the index predicate is satisfied.  This
requires a bit of reordering of operations so that we check the predicates
before doing any selectivity estimates, but shouldn't really cause any
noticeable slowdown.  Per a comment from Michal Politowski.