pgsql: Fix LookupTupleHashEntryHash() pipeline-stall issue.

Fix LookupTupleHashEntryHash() pipeline-stall issue.

Refactor hash lookups in nodeAgg.c to improve performance.

Author: Andres Freund and Jeff Davis
Discussion: https://postgr.es/m/20200612213715.op4ye4q7gktqvpuo%40alap3.anarazel.de
Backpatch-through: 13

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7f5f2249b27a46a4d91d6be5aff188ca67719fa7

Modified Files
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src/backend/executor/execGrouping.c       |  29 ++++--
src/backend/executor/nodeAgg.c            | 163 +++++++++++++++---------------
src/backend/executor/nodeRecursiveunion.c |   4 +-
src/backend/executor/nodeSetOp.c          |   4 +-
src/backend/executor/nodeSubplan.c        |   4 +-
src/include/executor/executor.h           |   2 +-
6 files changed, 105 insertions(+), 101 deletions(-)