[llvm] [AMDGPU][True16][CodeGen] add fake16 to gisel test (PR #138588)

[Joe Nash via llvm-commits]

Sisyph wrote:

Seems reasonable to me. It does remind me that we will have to get the True16 globalisel path working before we can rip out fake16. 


https://github.com/llvm/llvm-project/pull/138588