[llvm] Eliminating li of 0 into arg registers of unused arguments (PR #122741)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> This PR as-is is not about introducing poison to the DAG (that's the how, not the what). The actual operative change is the handling of poison for the width extensions.

These should not be mixed in. Introducing poison should be separate from any of this other stuff


https://github.com/llvm/llvm-project/pull/122741