[llvm] [NVPTX] Use v2.u64 to load/store 128-bit values (PR #136638)

[Artem Belevich via llvm-commits]

Artem-B wrote:

PTX apparently supports 128-bit ld/st/mov on sm_70+ and it looks like we've already landed bits and pieces of support for it last fall, while I wasn't looking. We'll probably need to plumb it through to support 128-bit atomics on sm_90, and it would simplify things here, too (though we'll still need these changes for older GPUs).

@AlexMaclean is that something already on your radar? I was about to start poking at the atomics (#122760), but will be happy if it's about to be done by NVIDIA. :-)

https://github.com/llvm/llvm-project/pull/136638