Clean up minor collation issues in indxpath.c.

Get rid of bogus collation test in match_special_index_operator (even for
ILIKE, the pattern match operator's collation doesn't matter here, and even
if it did the test was testing the wrong thing).
Fix broken looping logic in expand_indexqual_rowcompare.
Add collation check in match_clause_to_ordering_op.
Make naming and argument ordering more consistent; improve comments.