[CostModel] Use min/max intrinsics for vecreduce.min/max costs

This changes the costmodelling of the vecreduce.min/max nodes to use the costs
of the relevant min/max intrinsics instead of expanding them to compare and
selects. The getMinMaxReductionCost have changed to take a Opcode for the
relevant intrinsic, dropping the IsUnsigned and CondTy parameters as they are
no longer needed.

A follow up patch will add some basic fminimum/fmaximum costmodelling.

Differential Revision: https://reviews.llvm.org/D153547