pgsql: Improve sift up/down code in binaryheap.c and logtape.c.

Improve sift up/down code in binaryheap.c and logtape.c.

Borrow the logic that's long been used in tuplesort.c: instead
of physically swapping the data in two heap entries, keep the
value that's being sifted up or down in a local variable, and
just move the other values as necessary. This makes the code
shorter as well as faster. It's not clear that any current
callers are really time-critical enough to notice, but we
might as well code heap maintenance the same way everywhere.

Ma Liangzhu and Tom Lane

Discussion: https://postgr.es/m/17336-fc4e522d26a750fd@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a2ff18e89ff8f29677084bffd1e3de9ca6cd7224

Modified Files
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src/backend/lib/binaryheap.c | 70 +++++++++++++++++++++++-----------------
src/backend/utils/sort/logtape.c | 54 +++++++++++++------------------
2 files changed, 63 insertions(+), 61 deletions(-)