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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Now, I am even less convinced of the original PR. It had no tests, which is almost forbidden. There was an explicit request for tests.
> 
> Now, you are saying that `byref` is rarely used.

Yes. I believe AMDGPU is the only in tree user. Most of the code should just follow along with byval in terms of representation tracking 

> 
> Dead code with no tests? I still vote for revert.

No, this is not the policy for handling out of tree targets. This should not be reverted. 


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