pgsql: Fix matching of boolean index columns to sort ordering.

Fix matching of boolean index columns to sort ordering.

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

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0777f7a2e8e0a51f0f60cfe164d538bb459bf9f2

Modified Files
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src/backend/optimizer/path/indxpath.c | 46 ++++++++++++++++++++++++++++++
src/backend/optimizer/path/pathkeys.c | 35 ++++++++++++++++-------
src/include/optimizer/paths.h | 2 ++
src/test/regress/expected/create_index.out | 42 +++++++++++++++++++++++++++
src/test/regress/sql/create_index.sql | 15 ++++++++++
5 files changed, 129 insertions(+), 11 deletions(-)