[llvm] Use `FUNC_CODE_INST_ATOMICRMW_OLD` legacy code for bitcode writer. (PR #219277)

[Justin Bogner via llvm-commits]

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

LGTM. Might be worth stringifying the `INST_ATOMICRMW_OLD` code in the bitcode analyzer.

https://github.com/llvm/llvm-project/pull/219277