pgsql: Make better use of the new List implementation in a couple of pl

Make better use of the new List implementation in a couple of places

In nodeAppend.c and nodeMergeAppend.c there were some foreach loops which
looped over the list of subplans and only performed any work if the
subplan index was found in a Bitmapset. With the old linked list
implementation of List, this form made sense as accessing the Nth list
element was O(N). However, thanks to 1cff1b95a we now have array-based
lists, so accessing the Nth element has become O(1).

Here we make the most of the O(1) lookups and just loop over the set
members of the Bitmapset with bms_next_member(). This performs slightly
better when a small number of the list items are in the Bitmapset. Micro
benchmarks show that when the Bitmapset contains all or most of the list
items then the new code is ever so slightly slower. In practice, the cost
is so small that it's drowned out by various other things such as locking
the relations belonging to each subplan, etc.

The primary goal here is to leave better code examples around which benefit
better from the new list implementation.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAKJS1f8ZcsLVgkF4wOfRyMYTcPgLFiUAOedFC+U2vK_aFZk-BA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/efdcca55a3df27a12efb84a18bce6ea739927b80

Modified Files
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src/backend/executor/nodeAppend.c | 25 ++++++++++---------------
src/backend/executor/nodeMergeAppend.c | 14 +++++---------
2 files changed, 15 insertions(+), 24 deletions(-)