[llvm] [MachineCSE][AArch64] Coalesce subregister copies behind an opt-in target hook (PR #224593)

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

It's not clear from the regression test changes that this is an improvement. To me it looks like this is showing many regressions.

https://github.com/llvm/llvm-project/pull/224593