[IR] llvm::createMinMaxOp - create integer min/max intrinsics instead of icmp/sel

Based off D148215, when expanding a min/max reduction we should be creating min/max intrinsics directly instead of relying on instcombine to fold them back together.

This patch handles integer min/max cases. Hopefully we can add floating point support soon (at least for fastmath/nnan cases) - but we're missing some of the plumbing to pass the correct FMF to the intrinsic at the moment.

Differential Revision: https://reviews.llvm.org/D148221