[llvm] [AMDGPU] Allow lane-op lowering for illegal types (PR #114887)

[Jay Foad via llvm-commits]

jayfoad wrote:

I think this won't work for arbitrary illegal types like i999, right? It only works for i8 because we have an explicit `setOperationAction(ISD::INTRINSIC_WO_CHAIN, ... i8 ..., Custom)`. So update the description.

Otherwise seems fine to me.

https://github.com/llvm/llvm-project/pull/114887