Change GEQO fitness comparisons to consider disabled_nodes.

Prior to v18, the total cost of a plan was just one number, a
floating-point value. Commit e22253467942fdb100087787c3e1e3a8620c54b2
introduced a second component, the number of disabled_nodes at or
below that level of the plan tree. Unfortunately, I (rhaas) failed
to realize that GEQO extracts the cost from each candidate plan and
compares it directly as a measure of fitness.

This means that, at least in principle, it's possible for GEQO to
prefer a plan with more disabled_nodes and lower cost over one with
fewer disabled_nodes and higher cost, which is incorrect. It may be
possible for that to become a practical issue in v18, but it's much
more likely to become an issue in v19 due to pg_plan_advice. Hence,
since the straightforward fix breaks ABI compatibility, back-patch
only to v19.

Backpatch-through: 19
Discussion: https://postgr.es/m/CA+TgmoYmXy-jiP5qDhqNEiYFEBzQsArO6O2d9E8szNZqi1bePQ@mail.gmail.com

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