Teach simplify_boolean_equality to simplify the forms foo <> true and

Teach simplify_boolean_equality to simplify the forms foo <> true and
foo <> false, along with its previous duties of simplifying foo = true
and foo = false.  (All of these are equivalent to just foo or NOT foo
as the case may be.)  It's not clear how often this is really useful;
but it costs almost nothing to do, and it seems some people think we
should be smart about such cases.  Per recent bug report.