[llvm] [X86] Prefer `lock or` over mfence (PR #106555)

[Simon Pilgrim via llvm-commits]

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@@ -754,6 +754,10 @@ def TuningUseGLMDivSqrtCosts
 def TuningBranchHint: SubtargetFeature<"branch-hint", "HasBranchHint", "true",
                                         "Target has branch hint feature">;
 
+def TuningAvoidMFENCE
+   : SubtargetFeature<"avoid-mfence", "AvoidMFence", "true",
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RKSimon wrote:

Add tests for -mattr=+avoid-mfence/-avoid-mfence to atomic-unordered.ll

https://github.com/llvm/llvm-project/pull/106555