[llvm] [NVPTX] Add tex.grad.cube{array} intrinsics (PR #77693)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

@Artem-B, the changes to `llvm/test/CodeGen/NVPTX/surf-tex.py` add coverage for these intrinsics, is there additional testing you'd like?

https://github.com/llvm/llvm-project/pull/77693