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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REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3352e25e85b28ee1f21e0a759c0c49e399cb032a

Modified Files
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src/backend/utils/adt/int.c | 113 +++++++++++---------
src/backend/utils/adt/int8.c | 59 +++++++----
src/test/regress/expected/int2.out | 11 ++
src/test/regress/expected/int4.out | 21 ++++
.../regress/expected/int8-exp-three-digits.out | 21 ++++
src/test/regress/expected/int8.out | 21 ++++
src/test/regress/sql/int2.sql | 4 +
src/test/regress/sql/int4.sql | 8 ++
src/test/regress/sql/int8.sql | 8 ++
9 files changed, 198 insertions(+), 68 deletions(-)