[llvm] [AMDGPU] Fix pow with a negative base (PR #222248)

[Domenic Nutile via llvm-commits]

saxlungs wrote:

The disabled fma mix folding on True16 will be re-enabled by https://github.com/llvm/llvm-project/pull/205431 as it fixes the behavior, just a heads up.

https://github.com/llvm/llvm-project/pull/222248