[PATCH] heap_insert() and heap_update() optimization

Hello, hackers

I suggest the small attached patch that gives a bit of heap_insert() and heap_update() optimization
by reducing calls of BufferGetPage(buffer) into them.

I measured call time of these:
heap_insert(): avg origin 13394 ns, avg patched 12685 ns; perf increases +5.59%
heap_update(): avg origin 15728 ns, avg patched 13936 ns; perf increases +11.39%

This can be notable when there are handling many rows.
--
Regards,
Andrew K.