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.

This ought to be back-patched, but I'm going to see what the buildfarm
reports about the regression tests first.

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

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1f7cb5c30983752ff8de833de30afcaee63536d0

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(-)