Fix numeric_smaller, numeric_larger, float4smaller, float4larger,

float8smaller, float8larger (and thereby the MIN/MAX aggregates on these
datatypes) to agree with the datatypes' comparison operations as
regards NaN handling.  In all these datatypes, NaN is arbitrarily
considered larger than any normal value ... but MIN/MAX had not gotten
the word.  Per recent discussion on pgsql-sql.