[llvm] [AMDGPU] Make fast-fmaf an optional flag, defaulting to True for GFX9 (PR #161450)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

You should not be scanning through feature names, But also this is baked into the device properties. I do not think this should be optional 

https://github.com/llvm/llvm-project/pull/161450