[NVPTX] Add max/minimumnum to ISel (#155804)
Add direct support for the LLVM `maximumnum` and `minimumnum`
intrinsics for NVPTX, rather than lowering them to a sequence of
compare + select instructions.
The `maximumnum` and `minimumnum` intrinsics map directly to PTX
`max`/`min` instructions.
In future, the LLVM `maxnum`/`minnum` intrinsics might need some fix-ups
for sNaN handling added, but currently both `llvm.maxnum` and
`llvm.maximumnum` will map directly to PTX `max` instructions.
