Normally, if we have a WHERE clause like "indexcol = constant",
the planner will figure out that that index column can be ignored
when determining whether the index has a desired sort ordering.
But this failed to work for boolean index columns, because a
condition like "boolcol = true" is canonicalized to just "boolcol"
which does not give rise to an EquivalenceClass.  Add a check to
allow the same type of deduction to be made in this case too.

Per a complaint from Dima Pavlov.  Arguably this is a bug, but given the
limited impact and the small number of complaints so far, I won't risk
destabilizing plans in stable branches by back-patching.

Patch by me, reviewed by Michael Paquier

Discussion: https://postgr.es/m/1788.1481605684@sss.pgh.pa.us