Expand the 'special index operator' machinery to handle special cases
for boolean indexes. Previously we would only use such an index with WHERE clauses like 'indexkey = true' or 'indexkey = false'. The new code transforms the cases 'indexkey', 'NOT indexkey', 'indexkey IS TRUE', and 'indexkey IS FALSE' into one of these. While this is only marginally useful in itself, I intend soon to change constant-expression simplification so that 'foo = true' and 'foo = false' are reduced to just 'foo' and 'NOT foo' ... which would lose the ability to use boolean indexes for such queries at all, if the indexscan machinery couldn't make the reverse transformation.
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