pgsql: Fix daterange canonicalization for +/- infinity.

Fix daterange canonicalization for +/- infinity.

The values 'infinity' and '-infinity' are a part of the DATE type
itself, so a bound of the date 'infinity' is not the same as an
unbounded/infinite range. However, it is still wrong to try to
canonicalize such values, because adding or subtracting one has no
effect. Fix by treating 'infinity' and '-infinity' the same as
unbounded ranges for the purposes of canonicalization (but not other
purposes).

Backpatch to all versions because it is inconsistent with the
documented behavior. Note that this could be an incompatibility for
applications relying on the behavior contrary to the documentation.

Author: Laurenz Albe
Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/77f24ea19ab802bc9bc60ddbb8977ee2d646aec1.camel%40cybertec.at
Backpatch-through: 9.4

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f5f20deda33ba4146d68071193289f99c8a5d6c8

Modified Files
--------------
src/backend/utils/adt/rangetypes.c       |  4 ++--
src/test/regress/expected/rangetypes.out | 24 ++++++++++++++++++++++++
src/test/regress/sql/rangetypes.sql      |  4 ++++
3 files changed, 30 insertions(+), 2 deletions(-)