[CF 5584] v3 - moving some code out of explain.c

This branch was automatically generated by a robot using patches from an
email thread registered at:

https://commitfest.postgresql.org/patch/5584

The branch will be overwritten each time a new patch version is posted to
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on the master branch.

Patch(es): https://www.postgresql.org/message-id/CA+TgmoYtaad3i21V0jqua-fbr+CR0ix6uBvEX8_s6BG96abd=g@mail.gmail.com
Author(s): Robert Haas

Author: Commitfest Bot