[SLP] use reduction kind's opcode for cost model queries; NFC

[SLP] use reduction kind's opcode for cost model queries; NFC

This should be no-functional-change because the reduction kind
opcodes are 1-for-1 mappings to the instructions we are matching
as reductions. But we want to remove the need for the
`OperationData` opcode field because that does not work when
we start matching intrinsics (eg, maxnum) as reduction candidates.