pgsql: Minor optimizations based on ParallelContext having nworkers_lau

Minor optimizations based on ParallelContext having nworkers_launched.

Originally, we didn't have nworkers_launched, so code that used parallel
contexts had to be preprared for the possibility that not all of the
workers requested actually got launched. But now we can count on knowing
the number of workers that were successfully launched, which can shave
off a few cycles and simplify some code slightly.

Amit Kapila, reviewed by Haribabu Kommi, per a suggestion from Peter
Geoghegan.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/df4685fb0cad1c75970b6e8d0aacca4d03545e04

Modified Files
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src/backend/access/transam/parallel.c | 8 ++++----
src/backend/executor/execParallel.c | 2 +-
src/backend/executor/nodeGather.c | 18 +++++++-----------
3 files changed, 12 insertions(+), 16 deletions(-)