[SLP] replace local reduction enum with RecurrenceKind; NFCI

I'm not sure if the SLP enum was created before the IVDescriptor
RecurrenceDescriptor / RecurrenceKind existed, but the code in
SLP is now redundant with that class, so it just makes things
more complicated to have both. We eventually call LoopUtils
createSimpleTargetReduction() to create reduction ops, so we
might as well standardize on those enum names.

There's still a question of whether we need to use TTI::ReductionFlags
vs. MinMaxRecurrenceKind, but that can be another clean-up step.

Another option would just be to flatten the enums in RecurrenceDescriptor
into a single enum. There isn't much benefit (smaller switches?) to
having a min/max subset.