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

[Nikita Popov via llvm-commits]

================
@@ -0,0 +1,145 @@
+# RUN: llc -o - %s -O3 -mcpu=apple-m1 -mtriple=arm64-apple-macos | FileCheck %s
----------------
nikic wrote:

Can this be reduced to just a `-run-pass=machine-cp` test? Please also update update_mir_checks.py.

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