[llvm] [CodeGen] Preserve IMPLICIT_DEF for subregister joins (PR #218348)

[Karl Hylen via llvm-commits]

Hylen wrote:

> Please reply to this message to confirm that you have read these policies, especially the LLVM AI Tool Use Policy, and that any AI tool usage has been noted in the PR description.

I have read this and already added the AI information to the original description.

https://github.com/llvm/llvm-project/pull/218348