[llvm] MachineCopyPropagation: Do not remove copies preserved by regmask (PR #125868)

[Jinsong Ji via llvm-commits]

jsji wrote:

> Issue #126107 has a reproducer for this, so that can be added as a test. I've checked that applying this patch does fix that assertion.

Thanks! Added MIR test from #126107.

https://github.com/llvm/llvm-project/pull/125868