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

[Shilei Tian via llvm-commits]

shiltian wrote:

> This should demonstrate these do not select

They are demonstrated in https://github.com/llvm/llvm-project/pull/115190.

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