[SLP] Compute min/max scalar reduction costs using min/max intrinsics instead of expanded cmp+sel

By default these will expand back to cmp/sel, but some targets (X86) has optimized costs for scalar integer min/max patterns which are lower than the default expansion (pre-SSE41 is particularly weak for vector min/max support).

Differential Revision: [SLP] Compute min/max scalar reduction costs using min/max intrinsics instead of expanded cmp+sel