[llvm] [Scalar] Dedicated pass for identifying redundant operations on packed bytes (PR #146364)

[via llvm-commits]

zGoldthorpe wrote:

The `undef` deprecator detected is coming from an old test, though it also seems that `undef`s are only present in the FileCheck comments. The closest actual tests get are `%undef` vregs that are defined to be frozen poison values.

https://github.com/llvm/llvm-project/pull/146364