[llvm] [MachineCopyPropagation] Recognise and delete no-op moves produced after forwarded uses (PR #129889)

[Steven Michaud via llvm-commits]

steven-michaud wrote:

I'm giving up on creating a reduced testcase, at least temporarily. Sometime in the next few days I'll open an issue containing an `lldb` transcript of the [Firefox crashes that are easiest to reproduce](https://bugzilla.mozilla.org/show_bug.cgi?id=1995582), showing the copy propagation bug in action.


https://github.com/llvm/llvm-project/pull/129889