[llvm] [AMDGPU][True16][MC] add fake16 error and promote test (PR #135984)

[Joe Nash via llvm-commits]

https://github.com/Sisyph approved this pull request.

Adding something to the commit message as a reminder that these tests are about promoting the instruction encoding to 64 bits if the used registers are not encodable in the 32-bit form would be helpful. LGTM.

https://github.com/llvm/llvm-project/pull/135984