[llvm] Reland "[NVPTX] Add folding for cvt.rn.bf16x2.f32" (PR #116417)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> Looks like flipped args between LLVM and PTX is a common failure pattern. We should probably switch argument names from `a/b` to `hi/lo` where bit order matters or `e0/e1` for vectors to make it a bit easier catching such issues early next time. It's not fool-proof but would at least give a hint where particular argument is supposed to be placed.

Yea, inconsistent argument ordering in PTX does make things tricky. I've switched the patterns in this MR to use `lo` and `hi` as you suggested. 

https://github.com/llvm/llvm-project/pull/116417