pgsql: Fix DROP OPERATOR to reset oprcom/oprnegate links to the dropped

Fix DROP OPERATOR to reset oprcom/oprnegate links to the dropped operator.

This avoids leaving dangling links in pg_operator; which while fairly
harmless are also unsightly.

While we're at it, simplify OperatorUpd, which went through
heap_modify_tuple for no very good reason considering it had already made
a tuple copy it could just scribble on.

Roma Sokolov, reviewed by Tomas Vondra, additional hacking by Robert Haas
and myself.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/c94959d4110a1965472956cfd631082a96f64a84

Modified Files
--------------
src/backend/catalog/pg_operator.c           | 196 ++++++++++++++--------------
src/backend/commands/operatorcmds.c         |  20 +++
src/include/catalog/pg_operator_fn.h        |   2 +
src/test/regress/expected/drop_operator.out |  61 +++++++++
src/test/regress/parallel_schedule          |   2 +-
src/test/regress/serial_schedule            |   3 +-
src/test/regress/sql/drop_operator.sql      |  56 ++++++++
7 files changed, 239 insertions(+), 101 deletions(-)