[llvm] [AMDGPU][True16] Upstream more true16-related testing changes (NFC) (PR #208320)

[Domenic Nutile via llvm-commits]

saxlungs wrote:

I (hopefully) incorporated all of the feedback received so far and split this PR up into a stack that you can find here:

PR stack:
https://github.com/llvm/llvm-project/pull/209891
https://github.com/llvm/llvm-project/pull/209890
https://github.com/llvm/llvm-project/pull/209889
https://github.com/llvm/llvm-project/pull/209888

I will close this PR

https://github.com/llvm/llvm-project/pull/208320