Teach predtest about IS [NOT] <boolean> proofs

Hello,

I recently encountered a case where partial indexes were surprisingly not
being used. The issue is that predtest doesn't understand how boolean
values and IS <boolean> expressions relate.

For example if I have:

create table foo(i int, bar boolean);
create index on foo(i) where bar is true;

then this query:

select * from foo where i = 1 and bar;

doesn't use the partial index.

Attached is a patch that solves that issue. It also teaches predtest about
quite a few more cases involving BooleanTest expressions (e.g., how they
relate to NullTest expressions). One thing I could imagine being an
objection is that not all of these warrant cycles in planning. If that
turns out to be the case there's not a particularly clear line in my mind
about where to draw that line.

As noted in a TODO in the patch itself, I think it may be worth refactoring
the test_predtest module to run the "x, y" case as well as the "y, x" case
with a single call so as to eliminate a lot of repetition in
clause/expression test cases. If reviewers agree that's desirable, then I
could do that as a precursor.

Regards,
James Coleman