[SLP] rename reduction variables for readability; NFC

I am hoping to extend the reduction matching code, and it is
hard to distinguish "ReductionData" from "ReducedValueData".
So extend the tree/root metaphor to include leaves.

Another problem is that the name "OperationData" does not
provide insight into its purpose. I'm not sure if we can alter
that underlying data structure to make the code clearer.