[llvm] [CodeGen] Pass the atomic instruction to supportsUnalignedAtomics. NFC (PR #216621)

[Matt Arsenault via llvm-commits]

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

I'd prefer to not spread lowering hooks that depend on exact IR references, rather than properties. Particularly for the elementwise atomic case, it seems wrong to overload the meaning 

https://github.com/llvm/llvm-project/pull/216621