[llvm] [AArch64] Drop poison-generating flags in `genSubAdd2SubSub` combiner (PR #90028)

[David Green via llvm-commits]

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@@ -6924,14 +6924,22 @@ genSubAdd2SubSub(MachineFunction &MF, MachineRegisterInfo &MRI,
     assert((Opcode == AArch64::SUBWrr || Opcode == AArch64::SUBXrr) &&
            "Unexpected instruction opcode.");
 
+  uint32_t RootFlags = Root.getFlags();
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davemgreen wrote:

Perhaps use `Root.mergeFlagsWith(*AddMI);`, clear the NoSWrap/NoUWrap on it they use that in the new instructions?

https://github.com/llvm/llvm-project/pull/90028