Change pg_amop's index on (amopclaid,amopopr) to index (amopopr,amopclaid).

Change pg_amop's index on (amopclaid,amopopr) to index (amopopr,amopclaid).
This makes no difference for existing uses, but allows SelectSortFunction()
and pred_test_simple_clause() to use indexscans instead of seqscans to
locate entries for a particular operator in pg_amop.  Better yet, they can
use the SearchSysCacheList() API to cache the search results.