[llvm] [X86] Avoid useless DomTree in flags copy lowering (PR #97628)

[Shengchen Kan via llvm-commits]

KanRobert wrote:

> > Shouldn't the O0-pipeline.ll test fail since the dominator tree pass is no longer needed at -O0?
> 
> As a matter of fact they do, and I didn't notice, because these tests only runs in an assert build (do you know why it's not run in release builds?). Fixed now.

`--debug-pass` does not work when `LLVM_ENABLE_ASSERTIONS` is off.

https://github.com/llvm/llvm-project/pull/97628