pgsql: Improve handling of INT_MIN / -1 and related cases.

Improve handling of INT_MIN / -1 and related cases.

Some platforms throw an exception for this division, rather than returning
a necessarily-overflowed result.  Since we were testing for overflow after
the fact, an exception isn't nice.  We can avoid the problem by treating
division by -1 as negation.

Add some regression tests so that we'll find out if any compilers try to
optimize away the overflow check conditions.

Back-patch of commit 1f7cb5c30983752ff8de833de30afcaee63536d0.

Per discussion with Xi Wang, though this is different from the patch he
submitted.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/4387cc9ab4ad7e037df32e93c8435e34b5832cc5

Modified Files
--------------
src/backend/utils/adt/int.c                        |  108 ++++++++++----------
src/backend/utils/adt/int8.c                       |   90 +++++++++++------
src/test/regress/expected/int2.out                 |   11 ++
src/test/regress/expected/int4.out                 |   21 ++++
.../regress/expected/int8-exp-three-digits.out     |   31 ++++++
src/test/regress/expected/int8.out                 |   31 ++++++
src/test/regress/sql/int2.sql                      |    5 +
src/test/regress/sql/int4.sql                      |    8 ++
src/test/regress/sql/int8.sql                      |   11 ++
9 files changed, 233 insertions(+), 83 deletions(-)