pgsql: Remove assumptions that not-equals operators cannot be in any op

Remove assumptions that not-equals operators cannot be in any opclass.

get_op_btree_interpretation assumed this in order to save some duplication
of code, but it's not true in general anymore because we added <> support
to btree_gist. (We still assume it for btree opclasses, though.)

Also, essentially the same logic was baked into predtest.c. Get rid of
that duplication by generalizing get_op_btree_interpretation so that it
can be used by predtest.c.

Per bug report from Denis de Bernardy and investigation by Jeff Davis,
though I didn't use Jeff's patch exactly as-is.

Back-patch to 9.1; we do not support this usage before that.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/14f67192c25a130a354f3e3e6db3dfd49d2f5c86

Modified Files
--------------
src/backend/optimizer/util/predtest.c | 191 +++++++++++----------------------
src/backend/parser/parse_expr.c | 26 +++---
src/backend/utils/cache/lsyscache.c | 97 ++++++++++-------
src/include/utils/lsyscache.h | 12 ++-
4 files changed, 145 insertions(+), 181 deletions(-)