[llvm] [AMDGPU][GISel] Use datalayout alignment for buffer-load legalization (PR #95578)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

Still not sure if we should just use 4, but this matches the DAG behavior.

It would make more sense to move the s buffer intrinsics to use pointers and take the align attribute from the parameter 

https://github.com/llvm/llvm-project/pull/95578