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

[Steven Michaud via llvm-commits]

steven-michaud wrote:

Take a look at https://bugzilla.mozilla.org/show_bug.cgi?id=1995582#c20. I'm not entirely sure what you mean by "the issue fixed in April", but I think the second of my two workaround patches contains it in the block it comments out. So I don't think the "issue fixed in April" fixed this bug.

https://bugzilla.mozilla.org/show_bug.cgi?id=1995582 is 100% reproducible in Firefox local builds made with LLVM 21 (see particularly https://bugzilla.mozilla.org/show_bug.cgi?id=1995582#c16). So it wasn't hard to use `git bisect` to find this commit.

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