[llvm] [AMDGPU][GlobalISel] Allow bitcast of bf16 (PR #81674)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> It is there for a reason. You cannot really enable it without implementing global isel support for bf16. It will start failing silently. The point of this code to fail with an error. It will also start silently failing for multiple targets.

You can, bitcast doesn't count as a bfloat operation. As long as we fail on all the actual bfloat operations, it's OK.

https://github.com/llvm/llvm-project/pull/81674