[llvm] [NFC][AMDGPU] Guard FP8 related instructions properly (PR #115211)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> No need to have any test. It works as long as it doesn't break anything. This is more like a code reorganization.

>  The gfx9-4-generic target doesn't support those instructions, thus we need to update the guard properly.

This should demonstrate these do not select 


https://github.com/llvm/llvm-project/pull/115211