[NVPTX] Vectorize and lower 256-bit global loads/stores for sm_100+/ptx88+ (#139292)
PTX 8.8+ introduces 256-bit-wide vector loads/stores under certain
conditions. This change extends the backend to lower these loads/stores.
It also overrides getLoadStoreVecRegBitWidth for NVPTX, allowing the
LoadStoreVectorizer to create these wider vector operations.
See the spec for the three relevant PTX instructions here:
- https://docs.nvidia.com/cuda/parallel-thread-execution/#data-movement-and-conversion-instructions-ld
- https://docs.nvidia.com/cuda/parallel-thread-execution/#data-movement-and-conversion-instructions-ld-global-nc
- https://docs.nvidia.com/cuda/parallel-thread-execution/#data-movement-and-conversion-instructions-st