Summary:
The CUDA impelementation has long supported the `width` argument on its
shuffle instrucitons, which makes it more difficult to replace those
uses with this helper. This patch just correctly implements that for
AMDGPU and NVPTX so it's equivalent to `__shfl_sync` in CUDA. This will
ease porting.
Fortunately these get optimized out correctly when passing in known
widths.
(cherry picked from commit 2d8106cb5a505326d1da0f4461708ed44a0ac761)
1ee32d2234