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