[llvm] [AMDGPU] Use std::optional in InstCombine of amdgcn_fmed3. NFC. (PR #108223)

[Shilei Tian via llvm-commits]

https://github.com/shiltian approved this pull request.

Not sure what the real benefit is but it's fine.

https://github.com/llvm/llvm-project/pull/108223