Reduce the rescan cost estimate for Materialize nodes to cpu_operator_cost per

tuple, instead of the former cpu_tuple_cost.  It is sane to charge less than
cpu_tuple_cost because Materialize never does any qual-checking or projection,
so it's got less overhead than most plan node types.  In particular, we want
to have the same charge here as is charged for readout in cost_sort.  That
avoids the problem recently exhibited by Teodor wherein the planner prefers
a useless sort over a materialize step in a context where a lot of rescanning
will happen.  The rescan costs should be just about the same for both node
types, so make their estimates the same.

Not back-patching because all of the current logic for rescan cost estimates
is new in 9.0.  The old handling of rescans is sufficiently not-sane that
changing this in that structure is a bit pointless, and might indeed cause
regressions.

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