Previously these only worked via NVPTX-specific intrinsics.
This change will allow us to convert these target-specific intrinsics
into the general LLVM versions, allowing existing LLVM passes to reason
about their behavior.
It also gets us some minor codegen improvements as-is, from situations
where we canonicalize code into one of these llvm intrinsics.
For min/max I'd add few cases with an immediate value as the second argument.