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

[Steven Michaud via llvm-commits]

steven-michaud wrote:

This commit has has caused at least one crash bug, reported [here](https://bugzilla.mozilla.org/show_bug.cgi?id=1995582) and [here](https://github.com/NixOS/nixpkgs/issues/453372). I've confirmed that the crashes happen because one `mov w8,w8` instruction has been removed, leaving the upper 32 bits of `x8` uncleared. But Mozilla code is *very* complex, so I've been trying to write a reduced testcase.

I've made no progress at all. It might help if there was a `clang`/`clang++` flag I could use to activate *only* this particular optimization. Is there one? Any other suggestions?

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