[llvm] [MachineCopyPropagation] When the source of PreviousCopy is undef, we cannot replace sub register (PR #74682)

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+# RUN: llc -o - %s -O3 -mcpu=apple-m1 -mtriple=arm64-apple-macos | FileCheck %s
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DianQK wrote:

> Does `-stop-before=machine-cp` not work to get a machine-cp only test case?

That doesn't work. When I run machine-cp again, I don't get the expected result.

https://github.com/llvm/llvm-project/pull/74682