[llvm] [RegisterCoalescer] Materialize undef subrange PHI inputs (PR #204867)

[Dominik Montada via llvm-commits]

gargaroff wrote:

Sorry for only getting back to this now.

> Lately I've been thinking that the "optimization" to erase IMPLICIT_DEF is not worth it, and we should retain them for any register with sub registers

This was the golden comment that simplified the entire fix to just a couple of lines. My colleague has taken on the work and is preparing a new PR right now. I will therefore go ahead and close this one in favor of the much simpler version.

https://github.com/llvm/llvm-project/pull/204867