[SLP] use reduction kind's opcode to create new instructions; NFC

Similar to 5a1d31a28 -
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.