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

[Jay Foad via llvm-commits]

jayfoad wrote:

> > Not sure what the real benefit is
> 
> I find it much cleaner to return things via the return value of a function, instead of modifying one of its arguments.

I'd also be happy with returning `nullptr` on failure, instead of using `optional`.

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