Codomain

The Codomain the node carries where it differs from its type's
default, and null where it does not. Part of the node's identity,
deliberately. An e-class is the graph's assertion that its members are equal, and
abs(x) is not equal to domain(abs(x), Any) — they evaluate differently —
so two nodes differing only here must hash apart, or the graph unions values that are
not the same value. This used to live in a side table keyed on shape alone; both then
landed in one class and extraction returned whichever was inserted last, and
Rebuild dropped it altogether. ENodeIdentityTest holds all three.