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

[via llvm-commits]

AtariDreams wrote:

> @AtariDreams Reverse ping~
> 
> I met a problem cased by #86973 which was confused by `IMPLICIT_DEF` in this pass, and checked this patch can fix it. I put a commit to add the missing `found = true;` and optimize a bit. Please take a look, thanks!

Thank you! I just made a few changes, and I think this is ready now!

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