[llvm] Reland "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" (PR #168353)

[Benjamin Maxwell via llvm-commits]

https://github.com/MacDue commented:

I think the new fixes look good :+1: 

I just have some general questions/nits about the initial patch (as I didn't review it before, and it's a fairly tricky change). If @arsenm is happy with the change as-is, you can skip my suggestions. 

For the PR description, I think it'd be good to include the initial description, since that'd help explain the change in a git blame. Also, not sure if the commit references should be included, since they are not commits in the LLVM repo. Maybe just explain the changes from the initial PR directly in the description.  

https://github.com/llvm/llvm-project/pull/168353