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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> **Question for you both:** should this PR (a) stay as the targeted X86 stopgap until the general mechanism lands — it does fix the concrete miscompile from #160217 today — or (b) be retooled into the general mechanism directly? Happy to do either; if (b), I'd de-scope the `.td` changes here and build the opt-in plumbing in a fresh PR.

I think this PR is negative progress that won't be removed for years, so I'd rather go for the real solution 


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