[llvm] [AMDGPU][GlobalISel] Add register bank legalization for buffer_load byte and short (PR #167798)

[Philip Ginsbach-Chen via llvm-commits]

ginsbach wrote:

I'm seeing test failures in `LLVM.CodeGen/AMDGPU/GlobalISel/buffer-load-byte-short.ll` on my entirely unrelated PR: https://github.com/llvm/llvm-project/pull/171524#issuecomment-3647466264

https://github.com/llvm/llvm-project/pull/167798