[llvm] Revert "[GlobalISel] Add support for lowering byref attribute" (PR #96784)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> If you express interest in `byref`, then restart the PR with an AMDGPU test.

The AMDGPU usage of this is not going to be kludged through the DAG calling convention machinery, so will not use this 

> 
> `out of tree targets.`does not free you from tests.

It does if there is no way to test it with an in-tree target, especially for something small and mechanical like this. I'm rejecting this revert request 

https://github.com/llvm/llvm-project/pull/96784