Postpone calculating total_table_pages until after pruning/exclusion.

The planner doesn't make any use of root->total_table_pages until it
estimates costs of indexscans, so we don't need to compute it as
early as that's currently done.  By doing the calculation between
set_base_rel_sizes and set_base_rel_pathlists, we can omit relations
that get removed from the query by partition pruning or constraint
exclusion, which seems like a more accurate basis for costing.

(Historical note: I think at the time this code was written, there
was not a separation between the "set sizes" and "set pathlists"
steps, so that this approach would have been impossible at the time.
But now that we have that separation, this is clearly the better way
to do things.)

David Rowley, reviewed by Edmund Horner

Discussion: https://postgr.es/m/CAKJS1f-NG1mRM0VOtkAG7=ZLQWihoqees9R4ki3CKBB0-fRfCA@mail.gmail.com

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