[llvm] [X86] Prevent NUW flag forwarding in ADD->SUB peephole (PR #201612)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> * Add an **opt-in marker** for patterns where flag transfer is provably semantics-preserving — e.g. a TableGen `let CopyPoisonFlags = 1` on the pattern (similar in spirit to `SDNP*` properties), so each target has to claim flag preservation explicitly rather than getting it silently.

I suppose this is one way, but it's not great. It's not really a pattern level property. How do you map the control when there are multiple input nodes or output instructions?

https://github.com/llvm/llvm-project/pull/201612