Improve match_special_index_operator() to recognize that LIKE with an

exact-match pattern (no wildcard) can be index-optimized in some cases where a
prefix-match pattern cannot; specifically, since the required index clause is
simple equality, it works for regular text/varchar indexes even when the
locale is not C.  I'm not sure how often this case really comes up, but since
it requires hardly any additional work to handle it, we might as well get it
right.  Motivated by a discussion on the JDBC list.