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

[Phoebe Wang via llvm-commits]

phoebewang 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!

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