[llvm] [X86] X86LowerTileCopy: Find dead register to use to prevent save-reload of tile register (PR #83628)

[Phoebe Wang via llvm-commits]

https://github.com/phoebewang commented:

Thanks @nikic!

I didn't think `UsedRegs.stepBackward(MI);` is such expensive. I think we can early out with below code. How can we check it before commit?

https://github.com/llvm/llvm-project/pull/83628